Cecil, one of Zimbabwe's most famous lions, rests in the wild before he was killed by American Walter Palmer. Photo by Zimbabwe Parks and Wildlife Management Authority via European Pressphoto Agency

Keiko, the famous killer whale, in the Oregon Coast Aquarium in Newport, Ore., June 1998. Keiko starred in the film "Free Willy" and died swimming free in Norway in 2003. Photo by Don Ryan, AP

Panda cub Bao Bao's birth reflects a hopeful rise in numbers of giant pandas, a species that remains on the endangered list. Photo by Pablo Martinez Monsivais, AP

Cross-eyed o'possum Heidi is pictured as she sits in her enclosure at the Leipzig, Germany, zoo in December 2010. She died in September 2011. Photo by Hendrik Schmidt AFP/Getty Images

E.T. the walrus swims in Tacoma, Wash., in October 2011. The much-loved 33-year-old walrus died after surgery at the Point Defiance Zoo & Aquarium in July 2015. Photo by AP