Great apes, and species in general, don’t get much rarer than the critically endangered Cross River gorilla (Gorilla gorilla diehli).
The evolutionarily unique frogs of Cameroon's Lake Oku have no tongues, claw-tipped toes and 12 full sets of chromosomes. What the Lake Oku clawed frogs (Xenopus longipes) don't have, however, is a lot of habitat in which to live.
Oh what a difference a century makes. At the beginning of the 20th century, an estimated one million black rhinoceroses from four different subspecies roamed the savannas of Africa.