This would be much easier to answer if it were not for the pesky term, "Medieval." It is easy to find evidence of traditions involving abduction in early modern sources, but reaching back too far becomes problematic. That said, the classic migratory legend of "The Changeling," classified by the Norwegian, Reidar Th. Christiansen as ML 5085, is widespread throughout Northern Europe and even appears in the writings of Martin Luther. It even has an analogue (although involving witches) in the Roman source, The Satyricon.

Given all of this, it is easy to surmise that it was likely widespread in Northern Europe during the medieval period, but to keep us on a solid path, let's consider the traditions as they manifested during that early modern period (and project backwards when and if we are courageous enough!).

The fairies - the largely social supernatural beings of Northern Europe - were referred to with a variety of terms reflecting the many languages involved. For convenience, we will describe them generically as fairies. These powerful, dangerous supernatural beings were attracted to people, whom they frequently sought to abduct. Although everyone was vulnerable, the focus of attention was largely on male infants and young women of reproductive age. Stories about their abductions - attempted or accomplished - are ubiquitous. Women were particularly vulnerable after birth, clearly reflecting the possibility that these women could suddenly decline in health and die - or appear to die. For this reason, a newborn and a newly delivered woman were often confined to a sealed house until they could be admitted to church - a baptism for the infant and a "churching" (a readmission to the congregation) for the woman.

Numerous other prophylactic's were employed to safeguard against abductions - we could have a lengthy answer dealing with just that aspect of this topic, but at your request, we are here to consider the real-life consequences of a belief in changelings - both for the infants and for adults (one does not normally see these accounts about old people - the least desirable from everyone's point of view!).

A tragic case of an Irish woman who was believed to have been abducted occurred with Bridget Cleary who in 1895 began to inexplicably decline in health. Her husband - under the advice of another man in the village - concluded that his wife had been abducted and that the fairies had left a "stock" an inanimate (probably wooden) object that seemed to be alive, but which would soon revert to its lifeless nature, although still resembling the dead wife. People who were believed to have been replaced in this fashion would have their stock buried, unbeknownst to relatives that it was only a magically imbued log. Michael Cleary consequently placed his fading wife in a fire with the hope that his real wife would be returned. She wasn't. He was tried and convicted of murder - a conflict between modern British law and traditional Irish beliefs and practices with the former gaining the upper hand.

This tells us a great deal about how the fairy suspects of abduction cases could be treated - and likely there were generations of undocumented misconduct throughout Northern Europe because of this tradition.

Infants represented a different issue: adults could speak for themselves if they were well enough to do so, so really, only a fading adult or someone who died inexplicably could he held as suspect. An infant, however, could not speak, and so one could have a living substitute fairy in one's midst, and dealing with that supernatural being presented a different circumstance than what one normally encountered with adults who were suspect.

According to the variants of the legends, people suspected an infant changeling when their baby failed to thrive or seemed changed in some way. In reality, such suspicion could be aroused by various conditions including genetics, disease, or malnutrition, many of which were not apparent until sometime after birth.

In the legends, mothers attended the changeling in one of several ways - all designed to lure the fairy abductors to return the real infant. This took the form of tricking the changeling to speak with some astonishing act - brewing beer in an eggshell or some such thing. Other ways included beating the changeling, placing it on hot coals or exposing it at night outside. Legends invariably end with the return of the real infant - one of the few cases where legends (accounts told generally to be believed) turned out well.

In reality, things were likely much different and, indeed, tragic. Documentation of real efforts to deal with a suspect infant in these ways is sparse. W. Y. Evans-Wentz in his monumental study, The Fairy Faith in Celtic Countries (1911) described examples of children who had been raised under the suspicion of being changelings, and these poor victims no doubt suffered from considerable abuse and ostracism.

Many folklorists have written on the possible "real" roots behind the changeling legend. Clearly the narrative is traditional - a story with its own life history not dependent on anything real, but real examples of infants that did not seem to thrive likely put wind in the sail of the legend, and the legend informed many parents about what they should do in these circumstances.

This likely resulted in many instances of infanticide - and the pain involved (for parents and infant) is difficult to imagine. Fortunately (or not) we are left to imagine the possibilities more than to know: accounts of real instances were quickly subsumed into the tradition of the migratory legend, so it is typically not possible to "see" a clear case, as we can with the example of Bridget Cleary.

So, to attempt to answer your question: did the legend of the changeling and other efforts to abduct people by fairies result in real abuse if not murder? The likely answer is yes, although for the most part, we are left with speculation.

edit: typo

First, I don't feel very comfortable considering these beliefs to be medieval. The comment posted before mine on this matter is very important and a good read.

Fairy belief is largely tied to Ireland because fairy belief is incredibly important to Ireland's identity. Yes, there were people (and probably still are people today) who believe in fairies and the stories that go with them. When I was doing my research for my undergrad thesis (which was on fairy belief), I decided to stick close to the end of the 19th century, because this was when Irish nationalism and ideas of separating Ireland from the Union with Great Britain were really starting to gain power. This is where fairy belief became incredibly divisive.

Writers were using fairy belief to create romantic pictures of Ireland to sell to readers in Britain and elsewhere in the world (W. B. Yeats, Lady Gregory, Thomas Crofton Croker, etc.). Some of these authors were using Irish folk beliefs to push Irish nationalism and the fight for Home Rule (separation from Britain), some of them were using these stories to get rich because there was demand for them. All of these pictures of Ireland and folk belief were adapted-- changed for personal reasons in some cases (Thomas Crofton Croker collected a lot of stories about women being punished physically by fairies), changed because of literary tastes and by publishers, etc. These stories didn't say in Ireland, but circulated all over the place.

These stories were so popular in England that there were many people who believed in fairies based on what they had read about them. There were people trying to prove and disprove fairy belief all over the place, but most of these people weren't taking fairy belief literally. I like to think of this like people who believe in angels, they'll wear angel pins or look up stories about angels, but they're not out there trying to summon one through ritual. Just add some scientific method to some of it, and a lot of incidents that incite these beliefs (Cottingley Fairy photos, Spring-heeled Jack sightings, etc.).

A lot of the real beliefs and how they were told have been lost, not only did a lot of people die during tragedies like the Great Famine, but children didn't believe it was worth carrying on the legends that their parents and grandparents told them.

This is all to set up that, yes, there were people who literally believed in fairies and some people who still do.

One of the most important things to keep in mind about fairy belief is the storyteller-- who's telling this knowledge? Largely, it was women, sharing these stories with other women, children, etc. These stories held a coded language used among women, allowing them to talk about things that they couldn't talk about in front of male family members. Being "taken by fairies" could mean a lot of things, but in this code used among women, it meant that something wasn't right. Young women who dealt with heavy bouts of depression (not just depression, but chronic pain/illness) were often considered 'taken by fairies' or used the words themselves to explain that they felt out of touch with reality in some way.

These young women were usually around the age of 17-early 20s, marrying age. Young women in Ireland especially in the late 19th-century were often (but not always) marrying older men and had no way to connect with these older men on a deep personal level. They had been removed from their families and their mothers and told to become mothers themselves, they had responsibilities that they'd never had before. When these young girls felt too depressed to leave bed, their worried husbands might call in another woman to look over her as she heals. "Taken by fairies" might be coded words exchanged between these women.

This is largely believed to be the case with Bridget Cleary. She was a young woman murdered by her husband in 1895 because he took fairy belief too literally, not understanding that "taken by fairies" might be a metaphor for ailment or something else. Believing that she had been replaced by a changeling, he beat her, kicked her into the hearth fire, and dumped her body into a swamp. He then sat near the hill that he thought was a gateway to the Otherworld and waited for her to return. He was arrested there, waiting for her, and charged with murder. Their story was used by the British press to harm Ireland's fight for Home Rule (saying that they were too brutal to rule over themselves).

There were other cases where people took fairy belief literally. When I was doing my research, I found a case where an older woman had been given a toddler to look after. She thought the toddler had been taken by fairies, shoved an iron poker into the fire, and branded the child to drive the fairy away. I don't remember if the child survived.

In most of the cases I found, there were no specific references to disabilities. I'm sure that it happened, that young women overwhelmed with taking care of a child with needs they couldn't reach decided to get rid of a fairy, and got rid of their babies. The problem with finding these cases isn't just that they were under reported because they were mostly in rural areas, but because the police that were presiding over these were British forces brought into Ireland. These men didn't know what to write when dealing with a frenzied young woman talking about fairies, and sometimes these were just reported as drownings, babies falling into the hearth fire, etc.

(Doing a quick search, I found that a book was published on disabilities and fairy beliefs last year-- it's called Disfigured: On Fairy Tales, Disability, and Making Space by Amanda Leduc.)

As for your point about older people, I could absolutely see that being the case. I didn't find much on older women when I was doing my research, I mostly remember a story about a woman being attacked by a puca (a large shape-changing fairy, usually associated with the devil), published as "The Crookened Back." It was a story about how this older woman had no place in the community that she was a part of, that she was being punished by the puca for trying to surround herself with young people when she was old.

If you want to know more or have questions about anything, let me know and I'd be more than willing to do some research for you or send you any sources.

Contrary to what u/itsallfolklore said in their answer, we have a lot of evidence about the changeling motif in medieval Europe. Scholars tend to refer to changeling stories under the umbrella "the child substition motif", so I'll be using that throughout my answer too, especially since the English word "changeling" was not used in these stories until after the medieval period.

Theological discussions of changelings

One type of medieval literature which contains numerous references to the child substition motif is theologically motivated writings. One of the earliest instances is when William of Canterbury in 1172 makes a reference to a boy suffering from a wasting disease in his discussion of the miracles of Thomas Becket. In explaining the medical cause of the affliction (an ulcer on the lung), William says, "for no-one of sound mind credits the fabulous nonsense of the common people, who believe children to be substituted or transformed". William goes into no further detail about this belief of the "common people". The fact that the boy in question was suffering from a wasting illness is interesting to note though, since as we will see, the idea of a child who does not grow no matter how much milk they drink comes up again in early characterisations of the changeling.

Other early theological references to changelings are from the intellectual milieu of the University of Paris, one of the most important educational centres in the medieval West. C. F. Goodey and Tim Stainton go through these in detail in their 2001 article "Intellectual disability and the myth of the changeling myth". The first source they quote is William of Auvergne, an intellectual at Paris who makes reference to the motif in his De Universo of c. 1231-1240:

You should not overlook what is said about infants whom the convention [i.e. general theological opinion] calls cambiones, about which the most widespread are old wives' tales: that they are the children of demon incubi, substituted by female demons so that they are fed by them as if they are their own and are hence called cambiones, that is, cambici, as if swapped and substituted to female parents for their own children. They say that these are thin, always wailing, drinking so much milk that it takes four wet-nurses to feed one. They are seen to stay with their wet-nurses for many years, after which they fly away, or rather vanish.

Why does William bring up the cambiones in this text? Well, he attributes this story to the Spanish Dominicans, and goes on to debunk it. Academic rivalries were alive and well in the 13th century, and the Dominicans had made plenty of enemies by elbowing their way into the West's most hallowed educational institutions. The Dominicans had a very practical orientation in their scholarship since they were concerned with becoming preachers who could eradicate heresy in Latin Christendom.

In fact, William may well have been responding to the theological position of a particular Dominican, Jacques de Vitry, who also worked at the University of Paris. In his Sermones Vulgares, designed to inspire and instruct priests to be better preachers, he mentions the changeling story too. Dominicans were very concerned with impressing upon people that demons were literal realities that they had to be wary of, something which William of Auvergne considered to be overly superstitious. The Sermones Vulgares makes a brief reference to changelings: "children whom the French call chamium who suck dry many wet-nurses but nevertheless do not benefit or grow, but have a hard, distended belly". The story appears in a group of several others which are meant to prove that the Devil is real.

We therefore have to be very careful in interpreting William of Auvergne's reference to "old wives tales" as actual evidence of a folkloric source. William's reference to "old wives tales" is later in the text explicitly linked with the Dominicans, and he argues that the Dominicans are wrong to believe that such changeling children exist at all. He says the Dominican view that these children are anything but "malign apparitions" is "old wives' tales and senile delirium, not the truth". Are there really any old wives telling these tales, or is William of Auvergne simply comparing the Domincans to uneducated old women as a rhetorical insult?

The Dominicans continue to be prominent in our theological texts about changelings from the Middle Ages. The Dominican inquistors Henrich Kramer and Jakob Sprenger produced the 15th century text Malleus Maleficarum which included the following passage:

These children, which are commonly called cambiones, or in the German tongue Wechselkinder, are of three kinds. For some are always ailing and crying, and yet the milk of four women is not enough to satisfy them. Some are genearted by the operation of Incubus devils, of whom, however, they are not the sons, but of that man from whom the devil has received the semen as a Succubus, or whose semen he has collected from some nocturnal pollution in sleep ... And there is a third kind, when the devils at times appear in the form of young children and attach themselves to nurses. But all three kinds have this in common, that though they are very heavy, they are always ailing and do not grow, and cannot receive enough milk to satisfy them, and are often reported to have vanished away.

Nothing much has changed here since Jacques de Vitry's report three hundred years earlier. The main difference is that now the authors are very concerned about how the demons collect the sperm to give changelings form in the first place. You'll also perhaps have noticed that none of these referneces to changelings in theologically-driven medieval texts have anything to do with the child's intellectual capacity or physical irregularities. The recurring theme is of their insatiable thirst for breast milk. This is presented as evidence of their demonic origin since they suck the nursing mothers dry. When it comes to how to deal with these children, no advice is given -- they are simply said to vanish after a certain time. The reason for this is that these learned men were primarily concerned with debating whether changelings should be used as proof of the Devil's literal existence in the world. The draining-milk aspect of the insatiably thirsty infant is linked to the draining of semen from the succubus and to the generally negative impact of demonic influences on a soul. When it comes to Parisian elites, their interest is primarily a rhetorical and theological one, not a practical one.

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