Question
Why do first born donkeys have to be redeemed, but not firstborn of other nonkosher animals?
Answer
It would seem that the laws of the first born should apply to all unclean animal species, not just to donkeys (see Num. 18:15); but the Torah specifies (Ex.13:13) that they apply only to donkeys. The commentary of Ibn Ezra suggests that when the Jews were in Egypt, the only unkosher animals that they owned were donkeys, and the laws of the first born are applicable only to animals that Jews owned at that time.