We are learning in Maseches Kilayim, chapter 8, mishnah 2. The Torah commands: "Lo sacharosh b'shor uvachamor yachdav" - a person may not plow with an ox and a donkey together, and this is the prohibition of kilayim. The prohibition, however, is not limited to an ox and a donkey specifically; it includes any two different species.
We already encountered this principle in a mishnah in Maseches Bava Kama, which stated: "echad shor v'echad kol behemah" - whether an ox or any other kind of animal. One of the items listed there is kilayim, and that is the subject of our mishnah.
The pairings listed in the mishnah:
"Behemah im behemah" - one species of domesticated animal together with another species of domesticated animal, and an ox and a donkey are of course an example of this.
"Chayah im chayah" - a non-domesticated animal together with another non-domesticated animal.
"Behemah im chayah v'chayah im behemah" - a domesticated animal together with a non-domesticated animal, and the reverse.
"Temei'ah im temei'ah" - two non-kosher animals.
"Tehorah im tehorah" - two kosher animals.
"Temei'ah im tehorah u'tehorah im temei'ah" - a non-kosher animal together with a kosher animal, and the reverse.
It is worth noting that in the last member of each pairing - "chayah im behemah" and "tehorah im temei'ah" - the mishnah is repeating the very same case, since it is exactly the same law, and the repetition is not strictly necessary.
The three forbidden activities:
In each of these cases, wherever two different species are involved, three activities are forbidden:
"Lacharosh" - plowing with the two species together.
"Limshoch" - pulling, such as when they are pulling a wagon or any sort of load; they may not pull it together.
"L'hanhig" - even when nothing is being pulled at all, but they are tied together and a person is driving them. This driving too is included in the prohibition of kilei behemah.
In summary: the prohibition of kilei behemah is not limited to an ox and a donkey, but applies to any two different species - a domesticated animal with a domesticated animal, a non-domesticated animal with a non-domesticated animal, a domesticated animal with a non-domesticated animal, and likewise a non-kosher animal with a non-kosher animal, a kosher animal with a kosher animal, or a non-kosher animal with a kosher animal. In all of these it is forbidden to plow, to pull and to drive, for these two species may not work together.