The Torah mentions redeeming "the first-born of the unclean beast" in (Numbers 18:15), which could suggest that every unclean animal's firstborn must be redeemed. Camels, horses, dogs — any non-kosher creature. The Mekhilta steps in to narrow this sweeping reading.
"And the firstling of an ass you shall redeem" (Exodus 34:20) specifies the donkey. "And every firstling of an ass you shall redeem with a lamb" (Exodus 13:13) reinforces the point. The Torah did not say "every unclean beast." It said "an ass." The specific overrides the general.
The rabbis concluded: it is the firstborn of a donkey that you redeem, and not the firstborn of other unclean animals. The donkey alone among non-kosher creatures carries this unique sanctity.
Why the donkey? The midrash offers a historical explanation. When Israel departed Egypt, every Israelite loaded spoils of gold, silver, and garments onto donkeys. These animals literally carried the nation's redemption wealth on their backs. Because the donkey served Israel at the moment of their liberation, its firstborn was singled out for a special commandment — a perpetual reminder that even beasts of burden played a role in the great Exodus.