white horse vs whitecap

white horse and whitecap both are nouns.

Word	Noun	Adjective	Verb	Adverb
white horse	Yes	No	No	No
whitecap	Yes	No	No	No

As nouns, white horse and whitecap are synonyms defined as:

white horse and whitecap: a wave that is blown by the wind so its crest is broken and appears white

white horse (noun)	whitecap (noun)
a wave that is blown by the wind so its crest is broken and appears white	a wave that is blown by the wind so its crest is broken and appears white