To mark the day, learn more about some of the little-known but fascinating creatures among thousands listed as "endangered" or higher on the IUCN Red List.
Hawaiian Monk Seal, "Dog That Runs in Rough Water"
Locals in Hawaii have a special name for this seal because it resembles man's best friend. These seals are loyal to their young and may lose hundreds of pounds caring for them.
How many are left? About 1,200
How can they be helped? Help bust harmful myths about these seals, like the claim that their presence increases the likelihood of shark attacks. A large group of animal lovers have come together to form the Aloha Kanaloa Coalition, hoping to stop intentional killing of the animal.
Tree kangaroos, the unusual climbers
Unlike non-tree-climbing kangaroos, these unique critters have strong arms that are about the same length as their legs. There are 14 species of tree kangaroos, only some of which are endangered.
How many are left? Fewer than 2,500 Matschie's tree kangaroos
How can they be helped? Hunting is a major problem for these creatures. The World Wildlife Fund is combating this issue by maintaining protected areas and providing education about the risks.
Sumatran rhinos, the smallest of the rhinos
At less than 10 feet long and less than five feet tall, this is the smallest living species of rhinoceros. It is the only species of rhino in Asia with two horns, and it's critically endangered due to poaching and threats to its habitat.
How many are left? Fewer than 250
How can they be helped? You can help by buying sustainable wood, paper and palm oil products so their habitat is protected. Also, never buy rhino horn products.
Bonobos, "the pygmy chimp"
Usually under three feet tall, this species was the last great ape to be recognized by scientists. The human-like creatures are tied with chimpanzees for being our closest relative, as they share 98.7 percent of their DNA with humans.
How many are left? Fewer than 50,000
How can they be helped? You can adopt a bonobo through the World Wildlife Fund.
To learn more about the thousands of other endangered species and how you can help, visit the IUCN Redlist website or endangered.org