marmoset vs lion monkey

marmoset and lion monkey both are nouns.

Word	Noun	Adjective	Verb	Adverb
marmoset	Yes	No	No	No
lion monkey	Yes	No	No	No

As nouns, lion monkey is a hyponym of marmoset; that is, lion monkey is a word with a more specific, narrower meaning than marmoset:

marmoset: small soft-furred South American and Central American monkey with claws instead of nails
lion monkey: small South American marmoset with silky fur and long nonprehensile tail

Other hyponyms of marmoset include true marmoset, Cebuella pygmaea, pygmy marmoset, leoncita, lion marmoset, tamarin.

marmoset (noun)	lion monkey (noun)
small soft-furred South American and Central American monkey with claws instead of nails	small South American marmoset with silky fur and long nonprehensile tail