Mishnah Bechorot 6-7

If the Bechor has five legs or only three legs, we are specifically referring to the front legs of the animal. In this case, the front section may have three legs, or it may have one leg, with two additional legs in the back, resulting in a total of five or three legs. This type of blemish, along with other conditions such as if the animal's feet are round like a donkey, or exhibit characteristics of a shahul or a kasul, disqualifies the Bechor. Such animals may be slaughtered and consumed outside the Beit HaMikdash, as they are considered ordinary meat. To clarify, a shahul occurs when the thigh bone has slipped out of its normal position, which constitutes a blemish. A kasul is defined as when one of the thigh bones is attached to the body higher than it should be. Both of these conditions are regarded as blemishes and are problematic, leading to the disqualification of the Bechor.