What do you call a cow with no legs? (Ground Beef!) No, a cow! The absence of legs does not change the fact that the species is still a cow!
What do you call a DOG with no legs? (A dog?) It doesn't matter what you call it, as it's never going to come.
The joke plays on expectations. The first question sets up a wordplay punchline, but the answer is a literal rebuttal. The second question creates a similar expectation, but the punchline is a completely different, more absurd answer, which is anti-climactic.