Ragnar Lothbrok is an interesting case. He hovers just beyond the realm of history but is not quite wholly consigned to the realm of legend and story as many of the other legendary kings are. Indeed he, or at least some Norse leader named Ragnar, is attested in sources outside of the sagas and other Scandinavian works. While it would of course be a stretch to believe that a reference to a possible Ragnar in Frankish sources corroborates the sagas and stories about the man, it is also at the same time impossible to dismiss.

Now just to be clear, I am not saying that there was a Ragnar Lothbrok responsible for all the deeds that he is credited with in a variety of sources, to say that it stretches credibility is a bit of an understatement. Attempting to string together a comprehensive and complete account of his life from all of the disparate sources only strains its credibility that much more.

I'm going to focus on the legendary figure, as attested in a variety of Norse sources and not attempts to try and formulate a "historical" Ragnar, since this question is about folklore.

The majority of the stories about Ragnar come to us today through sagas or through Saxo Grammaticus's Gesta Danorum both of which are later compositions. The Gesta Danorum dates to the 12th or 13th centuries, a similar time frame as the sagas. Meaning that neither are contemporary sources and neither can be trusted to provide a simple retelling of events and deeds. It is telling I think, that earlier (and perhaps ironically more reliable sources do not mention him by name), thinking mainly of the Anglo-Saxon chronicle. On the English side of this story, he really does not make an appearance in any of the contemporary accounts, and he only pops up in the written record later, in Scandinavia. His sons do make appearances in contemporary English sources however.

Now it would be a little out there to claim that Ragnar was only thought up later in order to explain the presence of a whole bunch of angry Norsemen in England in the mid 9th century. That said however, sources that are unambiguous in their depiction of Ragnar Lothbrok all date to much later in the medieval period than when he was supposedly active. He of course had to come from somewhere, and it unlikely he was a Medieval meme created for one sources and subsequently adopted by a whole host of others. It seems far more likely that he was an amalgamation of historical figures and filtered through literary tropes and ideas on what role he should fill.

For example. Ragnar is famously murdered by King Ælle in a snake pit. This sort of an end is not unique to Ragnar and his found in a story where Attila the Hun inflicts a similar punishment on one of the main characters. (The Atli in the Volsung Saga). It is also a literary bookend to his life and career, which according to Saxo Grammaticus really took off after he hunted down two troublesome serpents. He also functions as a sort of law giver, and according to Saxo Grammaticus again, instituted the trial by jury.

Now as to why so many Scandinavian polities claimed descent through him, and how reputable these claims are, is a bit beyond me. I've read in quite a places that it was an easy means to legitimizing rule, and not significantly different than many other dynasties claiming descent from Odin (or Wotan in Anglo-Saxon England). I don't find this an entirely satisfactory answer, but I am unaware of any other more fleshed out reasons.

tl;dr, sources about him all date to roughly 1300, give or take a few decades. However there were likely earlier accounts that formed the basis of these accounts.