Animals on Shabbos
14. It is forbidden to exert oneself on Shabbos, solely for the sake of [increasing] an animal's physical comfort ("Ta'anug"); therefore, it is not permitted to hang a bag or container of feed on an animal, because that [contraption] is purely for the animal's general comfort, in not having to stretch out its neck.
However, it is permitted to hang a bag of feed on the necks of calves and ponies, because they have short necks and therefore experience difficulty eating from the ground (1); this applies in a courtyard, but it is forbidden to allow them to go out to the public domain carrying it, since it is considered a burden (2).
(1) That is, one is allowed to exert oneself to alleviate discomfort, but not to increase the comfort level of the animal, because it is considered unnecessary exertion ("Tircha Shelo Le'Tzorech").
(2) Shulchan Aruch 305:10