One quick difference is that a big dog who eats too much will simply throw up
and go on its merry way. The horse will just die. That, to me, is the biggest
caveat about horse keeping, but it's much less a problem if you can free
range. Mine do fine on grass and never eat too much in spring because they're
on it year round and get acclimated as the green comes in. If a horse kept in
a small paddock gets into the field of spring grass and eats at will for
hours, it could have serious consequences if it doesn't kill them.