charley horse vs charley-horse

charley horse and charley-horse both are nouns.

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

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

charley horse and charley-horse: a muscular cramp (especially in the thigh or calf) following vigorous exercise

charley horse (noun)	charley-horse (noun)
a muscular cramp (especially in the thigh or calf) following vigorous exercise	a muscular cramp (especially in the thigh or calf) following vigorous exercise