SCP-323 Wendigo Skull (Wendigo Transformation)

Thanks to Bro43man for recommending this video, which he describes as "one of those scp anime stories about a haunted deer skull that forces normal people to try it on their head; it then transforms them into a wendigo-type monster."

Sleepy Hollow (Wendigo Transformation)

This was a nice Halloween surprise. In episode 6 of second two ("And the Abyss Gazes Back") we're treated to a transforming wendigo, courtesy of a combination of make-up, a monster suit, and excellent CGI. The CGI works especially well because it serves as a bridge between two real figures, the human actor and the monster suit, resulting in a very convincing monster. (Are you taking notes Teen Wolf?)
After the transformation come two reversion scenes--the first quick and seen via reflection, the second longer. They're also notable because the actor who plays the wendigo is quite easy on the eyes, since he transforms back without any clothes on...

Hulk and the Agents of Smash (Wendigo Transformations)

This Marvel show features the Incredible Hulk as the captain of several other hulks and hulk-like freaks. In episode 10, "Wendigo Apocalypse," the team comes across a swarm of wendigos--anyone scratched by them automatically becomes a wendigo...
This clip is cut together from several scenes. In the first segment, the Hulk's friend Rick Jones becomes a Wendigo. In the next, Hulk becomes one too. But then he teams up with Wolverine to destroy the head Wendigo, causing all the other wendigos to return to normal.