yearling vs placental mammal

yearling and placental mammal both are nouns.

Word	Noun	Adjective	Verb	Adverb
yearling	Yes	No	No	No
placental mammal	Yes	No	No	No

As nouns, placental mammal is a hypernym of yearling; that is, placental mammal is a word with a broader meaning than yearling:

yearling: an animal in its second year
placental mammal: mammals having a placenta; all mammals except monotremes and marsupials

Other hypernyms of yearling include eutherian, eutherian mammal, placental.

yearling (noun)	placental mammal (noun)
an animal in its second year	mammals having a placenta; all mammals except monotremes and marsupials
a racehorse considered one year old until the second Jan. 1 following its birth
a young child who is just beginning to walk