trotter vs trotting horse

trotter and trotting horse both are nouns.

Word	Noun	Adjective	Verb	Adverb
trotter	Yes	No	No	No
trotting horse	Yes	No	No	No

As nouns, trotter and trotting horse are synonyms defined as:

trotter and trotting horse: a horse trained to trot; especially a horse trained for harness racing

trotter (noun)	trotting horse (noun)
a horse trained to trot; especially a horse trained for harness racing	a horse trained to trot; especially a horse trained for harness racing
foot of a pig or sheep especially one used as food