Bechorot 7: When Offspring are Questionably Kosher

We learn that if a kosher animal looks like a non-kosher animal, it is kosher as long as its father was kosher.  If the father was non-kosher, the offspring is non-kosher.  The Gemara wonders whether a non-kosher animal could impregnate a kosher animal.  Rabbi Yehoshua ben Levi states that this cannot happen (nor can an a large animal from a small one, a domesticated animal from an undomesticated one, an undomesticated animal from a domesticated one excepted in one case discussed by Rabbi Eliezer.

Perhaps the father animal was a kalut, one born of a kosher animal but without split hooves.  The rabbis discuss what should be done in such cases.  We can be certain of the kashrut of these animals, regardless of the appearance.  For example if the offspring does not look like the kosher mother but looks like a camel or a donkey.  If a fetus does not look like an animal at all; it looks like a bird or a lizard, it is not considered viable and thus not kosher.