What might a pony that eats too much grain or grass develop?

Lamenitis (founder). This is a complex problem involving the 'shock absorber' system in the hoof. You will notice the pony limping or not placing weight on a hoof(s). They can also become angry or stubborn if asked to work. A vet or farrier should take a look at the hoof in question. Until the Veterinarian can get there, soak your pony's affected hooves in cold water.

They can also colic, which is very painful and potentially lethal. Colic is where the intestines of the pony stop moving properly. There a four chambers that should be checked regularly (by ear) for gut sounds. If colic develops, the pony will tend to want to roll or lie down and look 'sad'. Keep the pony up and walking until a Veterinarian can get to the animal. Walking can help the bowels restart, but always call your Vet.