gymnastic horse vs buck

gymnastic horse and buck both are nouns.

Word	Noun	Adjective	Verb	Adverb
gymnastic horse	Yes	No	No	No
buck	Yes	No	Yes	No

As nouns, buck is a hyponym of gymnastic horse; that is, buck is a word with a more specific, narrower meaning than gymnastic horse:

gymnastic horse: a padded gymnastic apparatus on legs
buck: a gymnastic horse without pommels and with one end elongated; used lengthwise for vaulting

Other hyponyms of gymnastic horse include pommel horse, side horse, long horse, vaulting horse.

gymnastic horse (noun)	buck (noun)
a padded gymnastic apparatus on legs	mature male of various mammals (especially deer or antelope)
	a framework for holding wood that is being sawed
	a gymnastic horse without pommels and with one end elongated; used lengthwise for vaulting
	a piece of paper money worth one dollar

gymnastic horse (verb)	buck (verb)
	jump vertically, with legs stiff and back arched
	move quickly and violently
	resist
	to strive with determination