It has four legs, but goes on one foot (4)
I believe the answer is:
mule
This clue could be a double definition.
'it has four legs' is the first definition.
The definition suggests a singular noun which matches the answer.
'goes on one foot' is the second definition.
The answer and definition can be both animals as well as being singular nouns.
Maybe there's a link between them I don't understand?
'but' acts as a link.
Can you help me to learn more?
(Other definitions for mule that I've seen before include "Obstinate person?" , "Hybrid beast" , "Equine hybrid" , "The animal said to be stubborn" , "Donkey and horse cross" .)