He's imagined to have two arms and four legs (7)
I believe the answer is:
centaur
'he's imagined to have two arms and four legs' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
This is all the clue.
Can you help me to learn more?
(Other definitions for centaur that I've seen before include "Horsey person" , "Mythical hybrid" , "Mythical equine creature" , "Fabulous beast, half man and half horse" , "'Mythical creature, half man, half horse (7)'" .)