If we have an animal working with food items, such as an ox treading grain, we are not allowed to muzzle the animal. Rather, we are to permit it to eat while it works. This mitzvah applies to both kosher and non-kosher animals and to produce that is both still attached to the ground and already harvested. It only applies to animals; if someone puts a muzzle on a human worker, he would not be in violation of this mitzvah (though he would be some kind of a huge jerk).
The reason for this is that forcing an animal to work with food but not allowing it to eat while doing so is mean-spirited. It's cruel to the animal and God doesn't want us to be cruel. Hopefully, by encouraging us to treat even a dumb beast kindly, we will develop our trait of kindness with other people.
This mitzvah applies in all times and places. It is discussed in the Talmud in the tractates of Baba Kama (54b) and Baba Metzia (89a). It is codified in the Shulchan Aruch in Choshen Mishpat 338. This mitzvah is #219 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #188 of the 194 negative mitzvos that can be observed today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.