This excerpt from my recent The Folklore of Cornwall provides an early example of this sort of transformation:

Gervase of Tilbury (1150–1228) [in his] Otia Imperialia includes a story of a ‘dracs’, a supernatural water-being who can assume human form. This source attributes the story to an event that occurred in France’s Rhône Valley, but whether the account came from that location is not known. It describes a woman who was held captive in the creature’s palace. She served as a nurse for the son of the dracs. During her three years in the otherworld, she accidentally touched her eye after eating eel, and its fat caused her to be able to see the supernatural realm as it truly was. After being dismissed, the woman happened to be at a market where she saw her former employer. She greeted him. Upon finding out which eye gave her the ability to see him, he touched it, at which point she lost the capacity to see the supernatural. This early example of ML 5070 is of note not only for its age but also because it features the role of the nurse rather than the midwife.

This sort of transformation is not common, however, so it would be surprising on some level if a fantasy author had stumbled into this. In addition, this supernatural being spends most of his time as a man rather than a "dracs" - which appears to be, in this instance, a type of water dragon. More likely its recent manifestation was someone's independent invention. If it is - as you imply - common in fantasy literature, the many examples may derive from a single author's invention rather than an obscure source such as Gervase of Tilbury.

A definitive answer will require a precise definition of "dragon," but if we include East Asian "lung," a book by written in the early 1900s by a British scholar of Chinese folklore named Marinus de Visser. In chapter VII he has a section saying "the dragon's transformations are unlimited." He describes ancient Chinese tales of lung/dragons taking on human form, usually as an old man or a young women. He likens this to older Indian tales of naga, which are basically snake people who sometimes appear human and sometimes as snakes, and live in underwater cities. But naga are more dissimilar from Western dragons than lung are. De Visser devotes the early parts of the book to drawing connections and tracing the lineage, so to speak, between naga stories and lung stories.

"The Dragon in China and Japan" by M. de Visser

https://archive.org/stream/cu31924021444728#page/n51](https://archive.org/details/cu31924021444728)

I'm on mobile right now and will try to quote the relevant text (and dates) later when I'm at my computer.