hobby vs hobbyhorse

hobby and hobbyhorse both are nouns.

Word	Noun	Adjective	Verb	Adverb
hobby	Yes	No	No	No
hobbyhorse	Yes	No	No	No

As nouns, hobby and hobbyhorse are synonyms defined as:

hobby and hobbyhorse: a child's plaything consisting of an imitation horse mounted on rockers; the child straddles it and pretends to ride

Other synonyms of hobby include rocking horse.

hobby (noun)	hobbyhorse (noun)
an auxiliary activity	a child's plaything consisting of an imitation horse mounted on rockers; the child straddles it and pretends to ride
small Old World falcon formerly trained and flown at small birds	a topic to which one constantly reverts
a child's plaything consisting of an imitation horse mounted on rockers; the child straddles it and pretends to ride