Japanese spaniel vs toy dog

Japanese spaniel and toy dog both are nouns.

Word	Noun	Adjective	Verb	Adverb
Japanese spaniel	Yes	No	No	No
toy dog	Yes	No	No	No

As nouns, toy dog is a hypernym of Japanese spaniel; that is, toy dog is a word with a broader meaning than Japanese spaniel:

Japanese spaniel: breed of toy dogs originating in Japan having a silky black-and-white or red-and-white coat
toy dog: any of several breeds of very small dogs kept purely as pets

Other hypernyms of Japanese spaniel include toy.

Japanese spaniel (noun)	toy dog (noun)
breed of toy dogs originating in Japan having a silky black-and-white or red-and-white coat	any of several breeds of very small dogs kept purely as pets