wheel horse vs dray horse

wheel horse and dray horse both are nouns.

Word	Noun	Adjective	Verb	Adverb
wheel horse	Yes	No	No	No
dray horse	Yes	No	No	No

As nouns, dray horse is a hypernym of wheel horse; that is, dray horse is a word with a broader meaning than wheel horse:

wheel horse: a draft horse harnessed behind others and nearest the wheels of a vehicle
dray horse: horse adapted for drawing heavy loads

Other hypernyms of wheel horse include draft horse, draught horse.

wheel horse (noun)	dray horse (noun)
a draft horse harnessed behind others and nearest the wheels of a vehicle	horse adapted for drawing heavy loads