Halakhah к Кила́им 9:5

From the laws of the commandment is that which they, may their memory be blessed, said (see Sefer HaMitzvot LaRambam, Mitzvot Lo Taase 218 and Mishneh Torah, Diverse Species 9:7) that the obligation of this commandment from Torah writ is only with two species like the ox and the donkey - that one of them is pure and the second one impure, [such] that their natures are very distant from one another. But with two species that are [both] pure or impure - like an ox and a goat, or a donkey and horse - it is not forbidden from the Torah. Rather it is [only] rabbinically that any two species that would [constitute a case of] forbidden inter-breeding are forbidden - and both [domesticated] beasts and [undomesticated] animals are included in the prohibition. And it is the same whether one plows with them or seeds or pulls a wagon with them. And so [too,] if one [person] was sitting in the wagon and one was leading [them] - both of them are lashed, because his sitting in the wagon [also] causes the animal to pull (see Mishneh Torah, Diverse Species 9:9). And even if a hundred [people] led together, all of them are lashed. And [about] that which they said that it is permissible to do all work with a person and an animal, as it states, "with an ox and a donkey" - and not with a person and a donkey or a person and an ox (see Mishneh Torah, Diverse Species 9:10) - the explanation of the matter is according to the root that we mentioned. As the joining of an animal is not distressing for a person, since there is no connection with it at all. And behold, it is like he is doing work with a tree or a stone; and [so] it does not at all come into the designation that we spoke about. And also from the matter of the commandment is that which Rabbi Yitschak said at the end of Makkot 22a, that one who leads an ox of those disqualified from the consecrated is lashed - even though it is one body, the verse made it like two bodies to be lashed for. And the rest of its details are explained in the eighth chapter of Kilayim (see Tur, Yoreh Deah 297).