hindlimb vs hind leg

hindlimb and hind leg both are nouns.

Word	Noun	Adjective	Verb	Adverb
hindlimb	Yes	No	No	No
hind leg	Yes	No	No	No

As nouns, hind leg is a hyponym of hindlimb; that is, hind leg is a word with a more specific, narrower meaning than hindlimb:

hindlimb: a posterior appendage such as a leg or the homologous structure in other animals
hind leg: the back limb of a quadruped

hindlimb (noun)	hind leg (noun)
a posterior appendage such as a leg or the homologous structure in other animals	the back limb of a quadruped