Kilayim, Chapter One, Mishnah Six
This is the final mishnah which lists species that are close to one another and yet nevertheless still constitute kilayim. This time the mishnah deals with animals, which means that they may not be cross-bred, nor may they be yoked together or put together to perform any labor. The mishnah will discuss these prohibitions in greater depth in chapter 8:1-2.
I realize that some of these species cannot successfully cross-breed, and some are not domesticated and therefore dont perform work. It seems that the rabbis were theoretically interested in categorizing animals, even if there was no likely halakhic ramification. This may be as close as the mishnah gets to a zoological work, such as may have been written by ancient scientists like Aristotle.
A wolf and a dog, a wild dog and a fox, a goat and a deer, a gazelle and a ewe-lamb, a horse and a mule, or a mule and a donkey, a donkey and a wild donkey, even though they are similar one to the other, constitute nevertheless, kilayim one with the other.