trotting horse vs trotter

trotting horse and trotter both are nouns.

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

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

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

trotting horse (noun)	trotter (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