water deer vs mouse deer

water deer and mouse deer both are nouns.

Word	Noun	Adjective	Verb	Adverb
water deer	Yes	No	No	No
mouse deer	Yes	No	No	No

As nouns, mouse deer is a hypernym of water deer; that is, mouse deer is a word with a broader meaning than water deer:

water deer: largest chevrotain; of marshy areas of west Africa
mouse deer: very small hornless deer-like ruminant of tropical Asia and west Africa

Other hypernyms of water deer include chevrotain.

water deer (noun)	mouse deer (noun)
largest chevrotain; of marshy areas of west Africa	very small hornless deer-like ruminant of tropical Asia and west Africa