One may not redeem [a firstborn donkey] with a calf, an undomesticated beast, a slaughtered animal, a tereifah, a hybrid, or a koy. R’ Elazar permits [redemption] with a hybrid because it is a seh; but he prohibits [redemption] with a koy, because it is doubtful. [If he] gave it to a Kohen, the Kohen may not keep it until he sets aside a lamb or kid in its stead.