Viki Gryphon by scully
One of the first characters I ever made coming into the online fandom was a leopardess named Viki with a mysterious past that not even she, herself, knew anything about. But one thing about her that is included in her mystery is her ability to transform into mythical beasts such as gryphons, unicorns, dragons, etc. No one really knows how she got this ability except that it is all contained in the blue hairband she wears. Without it, she's just a normal leopardess, but with it she is immortal and dangerous even to herself.