I created standing_on_three_legs today. I've been thinking about it for some time. Personally, I intend to use it mostly for Pokémon, but it should work for normal animals too. I'm not sure if any name is good enough that, but basically the idea is that a four-legged creature is standing on three legs.
Admittedly, the name "standing_on_three_legs" could potentially have different meanings: an eight-legged creature standing on three legs, or a three-legged creature standing normally. I hate that, these are completely different things. But it's the best tag name so far, it's overall a good tag. (it would be a mouthful to create a very exact tag named four-legged_creature_standing_on_three_legs)
Credits for the tag name "standing_on_three_legs" goes to BrokenEagle98.
Zapdos suggested creating a forum post about it. Feel free to discuss if you want.