Olingo
Echidnae
Pangolin

i had to google an Olingo, what a cutie!

the Potoo

Fossa

Harlequin Ducks (ducks in general)
Orangutan
Anteater
Narwhal
Manatee
Sea Turtles
Bearded Vulture
Squid
Octopus
Walrus

Could name more but don't wanna fill up everything XD

velvet worms, nudibranchs, horseshoe crabs, hummingbird moths!
...idk, I like invertebrates :V if you don't like them, what also comes to mind for me are, fancy chickens (Polish are my fav breed personally) and spiny mice

I haven't seen Pine Martens around much.

Insects in general!

And of course, the obligatory plug for the collared lizard.

Lesser hedgehog tenrec.
Or
Just hedgehog

I need more elephants drawn in my life, but they're not THAT rare I guess

Actually rare animals:

Groundhogs
Hedgehogs (Sonic does not count)
Platypuses

I don't think I've seen many of the following species even though most of them are well known.
Moon rat
Viscacha
Vicuña
Coconut crabs
Crane flies
Emus
Frogs and toads generally
Golden mole
Russian Desmond
Slender loris
Saiga antelope
Squid
Sea slugs
Pacarana

Dik-diks and Duikers

Just about any Australia marsupial aside from the kangaroo, so
quokka,
chuddich,
quoll,
numbat,
dibbler,
bilby
just to name a few.