One that's very thick-skinned (5)

I believe the answer is:

rhino

(Other definitions for rhino that I've seen before include "Horned ungulate" , "One charging" , "African/Asian ungulate" , "old money?" , "wild beast" .)

I've seen this clue in The Guardian.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.