87:14 One may not hang a feedbag on an animal because this is only done for the animal's convenience. We are not allowed to unnecessarily exert ourselves on Shabbos for the mere convenience of animals. However, calves and ponies have short necks that make it difficult for them to eat from the ground, so one may hang feedbags on them in a courtyard; they may not go out wearing feedbags (even if placed before Shabbos - Mishnah Brurah 305:38) because it would be considered carrying a burden.
87:15 One may not toss grain to chickens on the damp ground because of the possibility that some grain will remain there and later germinate. (What is thrown on Shabbos must be enough to be eaten within two days, before it would have a chance to germinate - MB 336:33.)