complain vs croak

complain is not a noun while croak is a noun.

Word	Noun	Adjective	Verb	Adverb
complain	No	No	Yes	No
croak	Yes	No	Yes	No

As verbs, croak is a hyponym of complain; that is, croak is a word with a more specific, narrower meaning than complain:

complain (noun)	croak (noun)
	a harsh hoarse utterance (as of a frog)

complain (verb)	croak (verb)
express complaints, discontent, displeasure, or unhappiness	pass from physical life and lose all bodily attributes and functions necessary to sustain life
make a formal accusation; bring a formal charge	make complaining remarks or noises under one's breath
	utter a hoarse sound, like a raven