If I recall correctly, Guillaume was an Elezen heretic who used a magic spell (a "glamour" in the original definition of the word) to make himself look like a dragon. When you defeat him, he returns to his original appearance as an Elezen. It is unclear whether the enemies in Stone Vigil and Snowcloak are dragons returning to their original appearance using the "Dragon Blood" spell or humans transforming/glamouring themselves into dragon form.

Regardless of the truth of the matter, dragons in fiction have historically taken human form for one reason or another. All inter-species intercourse is technically bestiality to some degree. Hyur and Elezen sex is bestial. Hyur and Roegadyn is bestial. Hyur and human is bestial; they're not the same species! Hyur and lalafell is pedophilia though, you sickos.