"Endling" explained

YouTube link.

When animals die out, the last survivor is called an endling. It is a word of soft beauty, heartbreaking solitude, and chilling finality. The title was borne by Lonesome George, the last Pinta Island tortoise, after which George the snail was named. It unites Martha the passenger pigeon, Benjamin the thylacine, and Booming Ben the heath hen. It will eventually describe either Najin or Fatu, the two last northern white rhinos—both female, neither pregnant.

Endlings are avatars of loss. In the midst of Earth’s sixth mass extinction, these singular creatures embody the crisis facing our dwindling fauna—and our failure to avert it. By the time a species is down to its endling, it is functionally extinct. Caring for an endling can nonetheless serve as a final act of defiance, or perhaps contrition. Small wonder that the custodians of endlings often get very attached to them.

Lots more information in an Ed Yong article in The Atlantic.