הלכה על חולין 118:3

From the laws of the commandment is that every beast and animal that brings up the cud does not have teeth in its upper (rear) jaw. And every beast in the world that brings up the cud separates its hoof except for the camel. And all that separate its hoof brings up the cud except for the pig. And there are ten species of beasts and animals that are permitted: Three [domesticated] beasts that are well-known and they are the ox, the sheep and the goat; and seven species of [wild] animals that are explicit in Scripture - "The deer, the gazelle, etc." (Deuteronomy 14:5). And [just] like we need to know the signs of the fit (kosher) ones to differentiate them from the impure ones, likewise must we know which is a species of beast and which is a species of animal regarding the chelev (forbidden fat) - as the chelev of a beast is forbidden, but the chelev of an animal is permissible, as we we wrote above in the warning of chelev; and also as the blood of an animal requires covering, but not the blood of a beast. And the signs of the [wild] animal, as to how it is distinguished from a [domesticated] beast is not stated in the Torah. Rather, we learned from the heard tradition (Chullin 59b) that it is recognizable by its horns, as the horns of an animal are scaled, notched and circular: scaled like the horns of an ox; notched like the horns of a goat - such that the notch be absorbed in them (tight); and round like the horns of a gazelle. Therefore [with] any [being] that does not have these signs in its horns, a man must practice the prohibition of its chelev. And one who finds a [wild] animal without horns - such that he is not able to check its horns - if he recognizes it clearly by its shape, such as that which he is used to, it is permitted for him to trust his recognition. And it is not said that he has to check with the horns no matter what. And the rest of its details are in Chullin.