» Wed Jun 05, 2013 11:16 pm
I consider an alter ego to be an alternate persona that someone has, either one that was created by the original personality to serve a specific purpose, or a completely separate personality within the same physical body (though this second personality must be native to this body, not an outside entity possessing it). The former is usually seen in superhero stories (for example, Batman is Bruce Wayne's alter ego, Spiderman is Peter Parker's), but can be seen in other media (such as Walter White and Heisenberg in Breaking Bad), while the second is what we see in people with dissociative personality disorder (also known as multiple/split personality disorder), which we see in characters like Dr. Jekyll and Mr. Hyde, Harvey Dent and Two-Face, and most interpretations of Bruce Banner and The Hulk.
The beastform could be considered an alter ego in the truest sense (a personality completely distinct and separate from the original much like Dr. Jekyll and Mr. Hyde) if the afflicted individual had no agency over their actions when transformed, similar to how werewolves have been portrayed in various media. However, in TES, the beastform still has the humanoid personality in it, provided the individual hasn't been overwhelmed by the beastblood.