tree shrew vs pen-tail

tree shrew and pen-tail both are nouns.

Word	Noun	Adjective	Verb	Adverb
tree shrew	Yes	No	No	No
pen-tail	Yes	No	No	No

As nouns, pen-tail is a hyponym of tree shrew; that is, pen-tail is a word with a more specific, narrower meaning than tree shrew:

tree shrew: insectivorous arboreal mammal of southeast Asia that resembles a squirrel with large eyes and long sharp snout
pen-tail: brown tree shrew having a naked tail bilaterally fringed with long stiff hairs on the distal third; of Malaysia

Other hyponyms of tree shrew include pen-tailed tree shrew, pentail.

tree shrew (noun)	pen-tail (noun)
insectivorous arboreal mammal of southeast Asia that resembles a squirrel with large eyes and long sharp snout	brown tree shrew having a naked tail bilaterally fringed with long stiff hairs on the distal third; of Malaysia