horse pistol vs horse-pistol

horse pistol and horse-pistol both are nouns.

Word	Noun	Adjective	Verb	Adverb
horse pistol	Yes	No	No	No
horse-pistol	Yes	No	No	No

As nouns, horse pistol and horse-pistol are synonyms defined as:

horse pistol and horse-pistol: a large pistol (usually in a holster) formerly carried by horsemen

horse pistol (noun)	horse-pistol (noun)
a large pistol (usually in a holster) formerly carried by horsemen	a large pistol (usually in a holster) formerly carried by horsemen