Horses stifle(knee) can lock so they can sleep on their feet, sometimes the stifle can lock accidentally. I took him for a walk around the yard and it unlocked itself.
For those interested, the condition also appears to be called "Upward Patellar Fixation" and there's a good article on it here [ Ссылка ] with some solutions.
![](https://s2.save4k.ru/pic/pyL2JyxfSC4/mqdefault.jpg)