Bechorot 13 - Firstborn of a Kosher Animal
If one designated a sheep for the redemption of a firstborn donkey, and the sheep died, the owner is responsible to replace it with another lamb, just as a father of a firstborn boy is responsible to replace the redemption money, should be it lost before it reaches the kohen - this is the opinion of Rabbi Eliezer. However, the Sages say that the owner is not responsible for the replacement lamb, just as he is not responsible for the second tithe, which, if lost, need not be replaced.
If one buys the fetus of an idolaters cow or sells him his own cow's fetus - although he is not allowed to do so, or buys the cow jointly with an idolater - the cow is exempt from the law of firstborn, because the Torah said " all firstborn in Israel " - but not in other nations.
By contrast, the kohanim and Levites are included in the obligation of the firstborn of kosher animals. They were only excluded from the redemption of their sons and donkeys. Thus, a kohen who owns a cow, sheep or goat that gives birth to a male as it first offspring brings it as an offering.
Art: Willem Romeijn - Cow, Goats and Sheep in a Meadow