toy dog vs Canis familiaris

toy dog and Canis familiaris both are nouns.

Word	Noun	Adjective	Verb	Adverb
toy dog	Yes	No	No	No
Canis familiaris	Yes	No	No	No

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

toy dog: any of several breeds of very small dogs kept purely as pets
Canis familiaris: a member of the genus Canis (probably descended from the common wolf) that has been domesticated by man since prehistoric times; occurs in many breeds

Other hypernyms of toy dog include dog, domestic dog.

toy dog (noun)	Canis familiaris (noun)
any of several breeds of very small dogs kept purely as pets	a member of the genus Canis (probably descended from the common wolf) that has been domesticated by man since prehistoric times; occurs in many breeds