horse fly vs horsefly

horse fly and horsefly both are nouns.

Word	Noun	Adjective	Verb	Adverb
horse fly	Yes	No	No	No
horsefly	Yes	No	No	No

As nouns, horse fly and horsefly are synonyms defined as:

horse fly and horsefly: large swift fly the female of which sucks blood of various animals

Other synonyms of horse fly include cleg, clegg.

horse fly (noun)	horsefly (noun)
large swift fly the female of which sucks blood of various animals	large swift fly the female of which sucks blood of various animals
	winged fly parasitic on horses