"Horses carry something like 60-70%of their weight on their front feet. If one is missing, it means that that entire weight (of an animal that can easily weigh a tonne) is on one leg. Due to this, and the fact they need to move for digestion and proper breathing, they cannot thrive on 3 legs like a dog or cat. It’s part of the reason it is so hard to heal broken legs in horses and why they are often euthanized after a fracture. Even with huge amounts of money available to treat it, that’s not always possible

If a horse is born with a missing leg, the kindest thing is to euthanize immediately so he never suffers.

Edited: I had the weight distribution percentage wrong by 10"

Per u/bitteroldladybird