Bechoros 1:2-3

Bechoros 1:2

If a cow gave birth to something that resembles a donkey, or if a donkey gave birth to something resembling a horse, it is exempt from the law of a firstborn. This is because Exodus 13:13 and 34:20 each specify “firstborn of a donkey.” From the repetition we derive that it only applies to a donkey that gives birth to a donkey. When it comes to eating, if a kosher animal gives birth to something resembling a non-kosher animal it may be eaten but if a non-kosher animal gives birth to something resembling a kosher animal, it may not be eaten. This is because that which comes from a non-kosher animal is non-kosher, while that which comes from a kosher animal is kosher. If a non-kosher fish swallowed a kosher fish, the kosher fish may still be eaten, while if a kosher fish swallowed a non-kosher fish, the non-kosher fish may still not be eaten. This is because the smaller fish is not the product of the larger fish that swallowed it.

Bechoros 1:3

If a donkey that had not previously given birth gave birth to two males, the owner gives one lamb to the kohein (because one of these donkeys is certainly a firstborn requiring redemption). If the donkey gave birth to a male and a female, the owner puts one lamb aside for himself. (It is not given to the kohein out of doubt as to whether the male is the firstborn.) If two donkeys that never before gave birth give birth to two males, their owner must give two lambs to the kohein; if they gave birth to a male and a female, or two males and a female, he must give one lamb to the kohein. If the two donkeys bore two females and a male, or two males and two females, the kohein receives nothing (because there is not necessarily a firstborn male).