It's an intriguing concept though. Imagine fighting what appears to be a regular bandit who suddenly morphs into a huge shaggy white beast.
And a were-yeti isn't as silly as it sounds. A D&D character I once played managed to get cursed and became a were-donkey, our dungeon master had an odd sense of humor. And the transformations were more random and happened more often in times of stress such as combat, one minute my towering bearded axe-wielding barbarian is hacking away at a bunch of Orcs then suddenly its Eeeeaaawwwe and I'm this strange half human, half pack animal.
You don't get it. A Yeti is already a humanoid. A Were-Yeti is like a Were-Werewolf, or a Were-Imperial/Breton/Altmer. It is logicaly absurd. Because Were-Whatever is a cross between a humanoid and an
animal. You could have a Were-Turtle for example but you can't have were-whatever-that-is-a-humanoid-already. Not because it is silly, but because it conflicts the very definition.