Kilayim 8:3

If someone drives two different species of animal together, he receives forty lashes (39, really), as does the passenger in a wagon pulled by animals of different species. Rabbi Meir says that the passenger does not receive lashes. If two animals of the same species are pulling and a third animal of a different species is tied to their reins, it is prohibited.

Kilayim 8:4

If oxen are pulling a wagon, a horse may not be tied to the sides or the back; Libyan donkeys may not be tied to camels even though they are similar. Rabbi Yehuda says that one may work all offspring of a mare together even if the father is a donkey (i.e., a mule). All the offspring of a female donkey (jenny) may be driven together, even if the father is a horse (i.e., a hinny). The offspring of a mare and a jenny may not be driven together.