The Torah [Lev. 11] says that fish must have fins and scales to be kosher -- no need for more specifics. For birds, it has no general rule but specific listed birds are not kosher. Winged things that walk on all four are not kosher, except for the listed ones. Animals must have cloven hoofs and chew the cud to be kosher -- but then the Torah lists specific animals that are not kosher. Why do that when the general rule is sufficient? There is more seemingly needless repetition on the matter in Deuteronomy 14:4-5. (Nothing is superfluous in the Torah.)
2 Answers
See Chulin 59a:
For a Tanna of the school of R. Ishmael taught: It is written: The camel because it cheweth the cud. The Ruler of the universe knows that there is no other beast that chews the cud and is unclean except the camel; therefore the verse particularly stated ‘it’.
(Soncino translation, my emphasis)
For a Tanna of the school of R. Ishmael taught: It is written: And the swine because it parteth the hoof. The Ruler of the universe knows that there is no other beast that parts the hoof and is unclean except the swine; therefore the verse particularly stated ‘it’.
(Soncino translation, my emphasis)
It’s needed to prohibit eating animals with only one (or no) kosher signs.
The general rule only implies that they are forbidden with the force of a positive commandment; these verses are a proper negative commandment, incurring lashes. (See Sifra.)