If hippopotamuses have very thick and tough skin, how do wildlife veterinarians give them shots?

They use hippo-dermic needles.

Explanation: This joke is a pun on the word “hypodermic,” which refers to needles used for injections. The punchline humorously replaces “hypo” with “hippo” to fit the context of a hippopotamus, creating a clever wordplay that adds humor to the idea of giving a hippo shots with a “hippo-dermic” needle.