People are warned not to forget to give their animals a rest on the Sabbath, for it is written: “Six days shall you do your work, and on the seventh you shall rest so that your ox and your donkey may rest” (Exodus 23:12).
Therefore it is forbidden to place a burden upon an animal on the Sabbath, but one may take out the animal with a bridle upon it on the Sabbath, as this is not considered a burden. But a goat habitually tries to evade the bridle, and there is a fear that the bridle will remain in the owner’s hand and lead to him carrying it in the public domain on the Sabbath. Therefore the sages forbade leading a goat by bridle and reins on the Sabbath unless the owner had made a hole between the goat’s horns and stuck the bridle between them so the bridle does not come off. In this case one is permitted to take the animal out with a bridle. Rav Yosef raised a question: what is the rule if the bridle is tied to the billy goat’s beard? What are the issues under debate? On one hand, it should be permitted because the goat will not try to break free; doing so would pull at his beard and cause him pain. On the other hand, tying to his beard is not a strong tie, and the bridle may untie from the goat’s beard, remaining in the owner’s hand and leading him to carry it on the Sabbath. (The Talmud does not reach a decision on the matter.)
(Babylonian Talmud, Tractate Shabbat 52a)