It is forbidden, according to the laws of the Torah, to muzzle an ox while it is threshing wheat. (They used to separate the wheat from the chaff by having an ox tread on the wheat stalks.) Muzzling the ox would limit what he could eat while threshing, but one must allow the ox to eat the wheat which he is threshing, for it is written in the Torah “Do not muzzle an ox while it is threshing” (Deuteronomy 25:4).
The sages of the Talmud discuss the details and ask: what is the rule if the ox has a thorn in its mouth? Must the thorn be removed? In this situation, with a thorn in its mouth, the ox will not be able to eat and it might be considered as threshing with a muzzled ox, or it might be that since the thorn entered the mouth on its own and the owners did nothing to cause it, it is not considered muzzling the ox while it threshes and therefore one would be permitted to thresh using an ox unable to eat of the wheat due to a thorn in its mouth.
They continued their debate: what is the rule if there is a lion near the ox? The ox will be so fearful he will not manage to eat. Must the lion be run off or not? They also asked what the rule would be if a blanket is spread over the wheat and the ox treads on the blanket. In this case the ox does not see the wheat and perhaps the law does not forbid this, or perhaps this method of “muzzling” the ox is also forbidden. (These questions were not resolved.)
(Babylonian Talmud, Tractate Bava Metzia 90b)