Kilayim, Chapter 8, Mishnah 4:
"Ein koshrin es hasus lo l'tzidei hakaron v'lo l'achar hakaron" - one may not tie a horse to the sides of the wagon, nor behind it. What is the case? Cows or other animals are pulling the wagon, and a person wants to station a horse that is still in training alongside them or behind them, so that it should learn to pull, even though he has no actual need for it. The concern is that the horse may push the wagon together with the cows, and this would amount to the prohibition of kilayim - and therefore it is not permitted.
"V'lo es haluvdekim lag'malim" - one may also not tie luvdekim together with camels. What are luvdekim? A particular type of donkey, a donkey that comes from Lod, which is very large and strong and bears certain similarities to a camel. Because of this, one might have thought that it is a species of camel; but it is not, and therefore one cannot tie it together with camels and work with them.
The words of Rabbi Yehudah:
"Kol hanoladim min hasus, af al pi she'aviv chamor, mutarim zeh im zeh" - any mule born to a mare, even though its father is a donkey, meaning the combination of a female horse with a male donkey that produces a mule - these are permitted with one another. There is no need to be concerned that each one is half donkey and half horse, and that perhaps the donkey side of one is working together with the horse side of the other. Since the mothers of both are mares, they are permitted to be together and to work together.
"V'chein hanoladim min hachamor" - the same law is stated regarding those born to a female donkey; even though their father is a horse, they are permitted with one another, for that very same reason.
"Aval hanoladim min hasus im hanoladim min hachamor - asurim zeh im zeh" - those born to a mare, to a female horse, together with those born to a donkey, to a female donkey - are forbidden with one another, since they are considered two different species.
In summary: in this Mishnah we learned the prohibition of tying a horse to the sides of the wagon or behind it, lest it pull along with the cows; the prohibition of tying luvdekim - the large, strong donkey of Lod that somewhat resembles a camel - together with camels; and the view of Rabbi Yehudah, that those born to a mare are permitted with one another, and likewise those born to a female donkey are permitted with one another, but those born to a horse together with those born to a donkey are forbidden with one another, because they are two species.