The Torah is specific about how to redeem the firstborn of a donkey: "And every firstling of an ass shall you redeem with a lamb" (Exodus 13:13). The Mekhilta takes this precision seriously and builds a fence of exclusions around the commandment.
A lamb means a lamb. Not a calf. Not a domesticated animal of another species. Not a beast that has already been slaughtered. Not a treifah — an animal with a fatal organic defect that renders it unfit. Not a koi, that mysterious creature the rabbis could never definitively classify as either domesticated or wild. And not a kilayim — a hybrid born from crossbreeding two different species.
Each exclusion addresses a potential loophole. Someone might reason that any animal of equivalent value should suffice. The Mekhilta says no. The Torah specified a lamb, and the rabbis understood this to mean a living, healthy, purebred lamb from a flock — nothing else.
This meticulous approach reflects a broader principle in rabbinic law. When God specifies the means of fulfilling a commandment, substitutions are not permitted, no matter how logical they might seem. The firstborn donkey holds a unique status in the Torah because donkeys helped carry Israel's wealth out of Egypt. The redemption must be performed exactly as prescribed — with a lamb, and only a lamb.