rocking horse vs plaything

rocking horse and plaything both are nouns.

Word	Noun	Adjective	Verb	Adverb
rocking horse	Yes	No	No	No
plaything	Yes	No	No	No

As nouns, plaything is a hypernym of rocking horse; that is, plaything is a word with a broader meaning than rocking horse:

rocking horse: a child's plaything consisting of an imitation horse mounted on rockers; the child straddles it and pretends to ride
plaything: an artifact designed to be played with

Other hypernyms of rocking horse include toy.

rocking horse (noun)	plaything (noun)
a child's plaything consisting of an imitation horse mounted on rockers; the child straddles it and pretends to ride	an artifact designed to be played with