Bechorot 40 - Blemishes of the Tail, Testicles, and Feet
Further blemishes: the sheath (skin pocket) of the male member that was notched; the genitals of a female animal sacrifice that were notched; the tail notched in the bone or the tip of the tail exposed to the bone. Rabbi Elazar said, "The sheath is a blemish only if notched, but if completely missing, it will eventually grow back."
A firstborn that has no testicles is blemished. If it has one testicle, Rabbi Ishmael says that if it has two pouches, it is a sign that it has a second testicle, and one need not check. Rabbi Akiva says that one needs to set it on its buttocks and squeeze, and the second testicle, if it's there, will come down, otherwise, it is a blemish. Rabbi Yochanan ben Nuri disagrees and says that even a testicle that is present but does not descend is not a blemish.
An animal that has five legs or three legs is blemished, but if a hind leg is missing, then it is in addition a terefah and cannot be eaten anyway, even with a blemish. A slipped thigh is a blemish but if the sinews have disintegrated, it is in addition a terefah .
Art: Richard Ansdell - The Wounded Hound