I have possibly found out the reason why horse anatomy in ESO is weird, especially on the hind legs as many people have pointed out in the past.
I believe it wasn't a mistake per se, but it's just a consequence of sharing the same skeleton for both horses and deer, it all makes sense once you compare the models with the real-life counterparts:

It's clear that whenever they designed the skeleton they were trying to make something between a horse and a deer just so they could use the same rig for both, the reason people only call that out on horses is that we're so familiar with them that any inaccuracy becomes obvious.
So that's my conclusion, they're not actually intended to be shaped like horses, they're just a generic ungulate model—not really a horse, not really a deer, but close enough to both.
@MorvanClaude on PC/NA, don't try to trap me with lore subjects, it will work🦇