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So, for a land-dwelling mammal to be kosher, it must meet to two conditions at the same time: it must chew its cud (which means swallowing its food, partly digesting it in one of its stomachs, regurgitating it back to its mouth, then chewing it well, swallowing again and completing the digestion in the remaining stomachs) and have cloven, that is even-numbered, hooves.

And so, for example, a pig has cloven hooves, but doesn't chew its cud, which is precisely why it's not kosher. A camel, on the other hand, does chew its cud and it does have two toes on each foot, but the toes end with toenails rather than hooves, which makes the camel treyf as well. A hare also passes its food twice throught through its digestice digest ice system, but obviously has no hooves, so it's out. The only animals which do meet both criteria at the same time belong to the suborder ''Ruminantia'', to use modern taxonomic terminology. Apart from the species listed in the Biblical passage above, ruminants also include such creatures as: water buffalo, mouse-deer, bison, moose, wapiti and giraffe. But if we only count those ruminants which are traditionally raised for meat in Europe, then we're only left with cattle, sheep and goats.

I should note here that the notion of ritual cleanness has nothing to do with whether a particular animal is phisycally physically clean or dirty. Ritual cleanness isn't a moral caterogy category either, so it's not like "clean" animals are good and the "unclean" ones are evil. All animals, as God's creations, deserve the same kind of respect from humans. The only difference is that some of them are considered fit for Jewish consumption, while others are seen as kind of… gross.