Bikkurim, Chapter 2, Mishnah 10. This mishnah continues the discussion of the status of the koi, and this time it focuses on the ways in which the koi resembles a behemah (a domesticated animal).
The ways in which the koi resembles a domesticated animal:
"Chelbo asur k'chelev behemah" - as was mentioned in the previous mishnah, the chelev, certain specific fats, is forbidden to eat in a domesticated animal, while those very same fats in a wild animal are permitted. Since it is not known whether the koi is a domesticated animal or a wild animal, we must be stringent and forbid eating its fat like the fat of any domesticated animal. The mishnah does add a qualification, however: one is not liable to kareis for it, and one does not bring a sin offering for eating it unintentionally and without intent, since the entire prohibition stems only from the doubt.
"V'eino nilkach b'chesef ma'aser sheni le'echol biYerushalayim" - one may not buy a koi with ma'aser sheni money in order to eat it in Jerusalem as ordinary food. The reason: a domesticated animal is not purchased with ma'aser sheni money to be eaten as ordinary food, for the finest thing to do with such money is to bring the animal as an offering; a wild animal, by contrast, which is not fit to be offered, may be bought with ma'aser sheni money and eaten in Jerusalem as ordinary food. Because of the possibility that the koi is a domesticated animal, we must be stringent and forbid buying it with ma'aser sheni money for ordinary eating.
"V'chayav bazroa u'valechayayim u'vakevah" - one who slaughters a domesticated animal is obligated to give the kohen the foreleg, the jaw and the maw, and this obligation applies only to a domesticated animal and not to a wild animal. Since we must be stringent and treat the koi as a domesticated animal, these gifts must be given to the kohen from it.
The view of Rabbi Eliezer:
Rabbi Eliezer exempts the koi from the priestly gifts. His reasoning: since the matter stands in doubt, the well known principle applies - "the one who wishes to extract from another bears the burden of proof." The kohen, who wants to claim that the foreleg, the jaw and the maw of the koi are due to him, is the one who must prove that it is a domesticated animal and that the gifts belong to him. As long as he has not proven this, the owner of the koi can respond that in his assessment it is a wild animal, and therefore he is not obligated to give him the foreleg, the jaw and the maw.