Shapeshifting, Metamorphosis, Transformation, Jekyll/Hyde: How Multiple Personality is Represented in Mythology, Folklore, and Speculative Fiction.

“The idea of shapeshifting is present in the oldest forms of totemism and shamanism, as well as the oldest extant literature and epic poems, including works such as the Epic of Gilgamesh and the Iliad, where the shapeshifting is usually induced by the act of a deity…It remains a common trope in modern fantasy, children's literature, and works of popular culture…

“Popular shapeshifting creatures in folklore are werewolves and vampires …the Huli jing of East Asia (including the Japanese kitsune), and the gods, goddesses, and demons of numerous mythologies, such as the Norse Loki or the Greek Proteus…Examples of shapeshifting in classical literature include many examples in Ovid's Metamorphoses…” (Wikipedia).

Wikipedia’s article on Shapeshifting goes on for sixteen pages:

Table of Contents

Folklore and mythology

1.1 Greco-Roman

1.2 British and Irish

1.2.1 Celtic mythology

1.3 Norse

1.4 Other lore

1.4.1 Armenian

1.4.2 Indian

1.4.3 Philippines

1.4.4 Tatar

1.4.5 Chinese

1.4.6 Japanese

1.4.7 Korean

1.5 Folktales

Themes

2.1 Punitive changes

2.2 Transformation chase

2.3 Powers

2.4 Bildungsroman

2.5 Needed items

2.6 Inner conflict (e.g., Jekyll’s transformation to Hyde)

2.7 Usurpation

2.8 Ill-advised wishes

2.9 Monstrous bride/bridegroom

2.10 Death

Modern

3.1 Fiction

3.2 Popular culture

Psychologically speaking, much of the above shapeshifting, metamorphosis, and transformation in world literature, now and throughout history—not just Jekyll and Hyde—represents the personality switches of multiple personality.

https://en.wikipedia.org/wiki/Shapeshifting