wheeler vs dray horse

wheeler and dray horse both are nouns.

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

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

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

Other hypernyms of wheeler include draft horse, draught horse.

wheeler (noun)	dray horse (noun)
a draft horse harnessed behind others and nearest the wheels of a vehicle	horse adapted for drawing heavy loads
someone who makes and repairs wooden wheels
a person who rides a bicycle
the man at the outermost end of the rank in wheeling