The Gemara teaches that an animal that is born from a kosher animal that has the appearance of a non-kosher animal, is nevertheless kosher, assuming that the father was kosher, as well. If, however, the father was a non-kosher animal, the offspring would not be kosher. The assumption that a non-kosher animal could impregnate a kosher animal is questioned by the Gemara:
Can a kosher animal become pregnant from a non-kosher animal? For Rabbi Yehoshua ben Levi said: A non-kosher animal never becomes pregnant from a kosher one, nor a kosher animal from a non-kosher one, nor a large animal from a small one, nor a small one from a large one, nor a domesticated animal from an undomesticated one, nor an undomesticated animal from a domesticated one, except in the case discussed by Rabbi Eliezer and his disputants, where all say that an undomesticated animal can become pregnant from a domesticated animal.
In response the Gemara explains that the first case was when the father animal was a kalut – it was born of a kosher animal but did not have split hooves.
Occasionally an animal with split hooves will give birth to a creature that has a birth defect – its hooves are not split. Although such animals do not have the usual indicators of kashrut , nevertheless we can be certain that it is from the family of kosher animals. This would be true even if the newborn animal does not have the appearance of the kosher mother, e.g. it looks like a camel or a donkey; since we know that it was born from a kosher animal it is kosher and can be slaughtered. This is not the case where the fetus does not have the appearance of an animal at all, e.g., when it looks like a bird or a lizard.