Liberty, the National Zoo’s 21-year-old female scimitar-horned oryx, is dead, the Zoo said.

She was euthanized Thursday morning after months of declining health, according to the zoo.

Liberty. (Courtesy of Jessie Cohen, National Zoo)

Liberty arrived at the Zoo in 2004, the zoo said. She was born March 1, 1991, at the Denver Zoo, and lived longer than the usual 15-year lifespan of such animals.

Oryx, native to northern Africa, are now extinct in the wild. The horned mammals are adapted for desert life.

Two younger, male oryx are still on display at the Zoo.