All breedable mobs should heal before breeding, just like wolves.

A mob that is punched will never, ever recover by natural means. A pig punched once will retain that damage from the dawn of the first sunrise to the moment you say farewell to the world you have poured months of time into. A sheep poisoned only once will die if so much as brushed up against no matter how much care it receives, even after many hours, many children, and many consumptions of grass.

Now, whether mobs should heal naturally like horses is another question indeed, but it's very strange that they can't be healed by anything other than splash potions. Wolves, on the other hand, can be healed by feeding them their breeding food. Only when they are fully healthy, will they breed, which makes thematic sense too. I suggest that this behavior be copied to all animals, and perhaps with some way of indicating that your food was used to heal instead of to breed, like a particle effect.