Yesterday Nicole Luiken riffed on the concept of whether various magical creatures could be "approved" as shifters. See, there are really several ways of looking at this kind of metamorphosis from human to animal.
The concept of a were-creature, as in a werewolf, comes from the juxtaposition of the two root words - literally man-wolf, as " ). The world of creative imagination has taken off from there to create all kinds of were-creatures. But using this terminology constricts the creature to being a human/animal, as in they transform to one kind of animal only.
Nicole posed the initial question of when did dragons become shifters instead of purely mythological creatures? She (humorously) answers "at least eight years ago." I'm not sure whose book she's thinking of there, though it would be interesting to know!
My answer? Since at least the 3rd Century BCE, and probably a hell of a lot longer than that.
In old Celtic lore, shifters roamed the countryside. These were people (using the term loosely) who could transform into any creature at will. Sometimes they're found in association with the Fae, othertimes not. Spenser, in The Faerie Queen, takes these old tales and explores the metaphor of metamorphosis. (See what I did there?) It's a topic that has long fascinated me.
Twelve Kingdoms books, THE MARK OF THE TALA, I knew I wanted to have shifters. Real shifters, who could take multiple forms. Not all of the Tala can shapeshift, but many can, particularly closer to the source of magic in their realm. The most proficient can take multiple forms.
There are even staymachs - animals that can be trained to shift into other animals.
It makes for an interesting world with tons of potential. Just wait until you all get to read THE PAGES OF THE MIND. More than that I cannot say. :-)