Obstinate creatures trodden underfoot (5)

I believe the answer is:

mules

I'm a little stuck... Click here to teach me more about this clue!

'obstinate creatures' is the definition.
The definition and answer can be both animals as well as being plural nouns.
Maybe you can see an association between them that I don't see?

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for mules that I've seen before include "Hybrid offsprings of a male donkey and a female horse" , "Offspring of male donkeys and female horses" , "Beasts of burden" , "Stubborn horse-donkey animaly" , "Backless slippers" .)

I've seen this clue in The Irish News.

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.