"Extinction is tragic, especially if it is preventable," according to the report, which was released earlier this month. The paper highlights a dozen species listed as critically endangered, and therefore most at risk of extinction, by the International Union for Conservation of Nature.
Only identified as a separate species in 2004, the highly endangered Sumatran orangutan (above, a Sumatran orangutan in Kansas's Sedgwick County Zoo) may disappear before scientists can properly study the species.
Wild populations of the orangutan, found only in Aceh Province in southern Sumatra, Indonesia, have declined by 80 percent in the past 75 years. Only about 6,600 orangutans are left in Sumatra, according to the "Rarest of the Rare" report.
The animals are rapidly losing habitat to palm oil plantations. Young orangutans are also sometimes killed as pests or illegally captured for the international pet trade, experts say.
Photograph by Joel Sartore, National Geographic
The smallest of the cetaceans—the group of marine mammals that includes bottlenose dolphins—the vaquita is also the most threatened, according to the WCS report.
In its native waters of Mexico's northern Gulf of California, the 5-foot-long (1.5-meter-long) vaquita often drowns in fishing nets (pictured, a casualty in Baja California).
Breeding between the highly endangered Cuban crocodile (pictured) and its cousin the American crocodile is taking a bite out of the rarer reptile's chances for survival, experts say.
One of 12 species featured in the new Wildlife Conservation Society report "Rarest of the Rare," the 11-foot-long (3.5-meter-long) crocodile is also being hunted for meat in its native Cuba.
Only about 4,000 individuals are left in just two small areas of the island country, though experts suspect many of the animals are hybrids.
Photograph by Steve Winter, National Geographic Stock
A victim of the deadly chytrid fungus, the green-eyed frog (pictured) has plummeted to only a few hundred individuals in Costa Rica and Panama, according to the "Rarest of the Rare" report.
Habitat lost to logging and deaths due to agricultural chemicals have dealt additional blows to the 2.5-inch-long (6.5-centimeter-long) frog. Breeding the amphibian in captivity may be the species' last hope, according to the Wildlife Conservation Society.
Photograph by Roy Toft, National Geographic
One of the most endangered antelopes in Africa, the hirola, or Hunter's hartebeest (pictured in Kenya's Tsavo National Park), is found only in a small region near the Kenya-Somali border.
The approximately 600 remaining animals are in danger due to habitat loss from cattle farming and severe drought, among other threats, according to the Wildlife Conservation Society report "Rarest of the Rare."
Photograph by Werner Bollmann, Photolibrary
Florida Bonneted Bat
The largest bat in Florida, the Florida bonneted bat was thought to be extinct until 2002, when scientists found a small colony of the flying mammals in Fort Myers.
The 21-inch-long (53-centimeter-long) bat is one of the critically endangered species highlighted in the new "Rarest of the Rare" report. The species has declined to just about a hundred animals due to widespread loss of its preferred roosting sites in cliff crevices and tree cavities.
Photograph by Merlin Tuttle, Bat Conservation International
The national bird of the Caribbean island of Grenada, the pink-breasted Grenada dove (pictured) numbers fewer than 150, according to the Wildlife Conservation Society's new report "Rarest of the Rare."
Habitat loss and introduced predators including mongooses, cats, and rats have pushed the remaining birds into a small area of the island country. A ten-year recovery plan is underway to save the dove, according to WCS.
Photograph courtesy Bonnie Rusk, Grenada Dove Conservation Program
White-headed langurs (pictured), native to Cat Ba Island in Vietnam's Halong Bay, have declined by 98 percent in the past four decades, according to the Wildlife Conservation Society.
Hunters have killed the animals to meet demand for traditional Chinese medicines, and habitat destruction has also taken its toll, the "Rarest of the Rare" report noted. The remaining 59 langurs are split into a few isolated groups, which may lead to inbreeding, experts say.
Photograph from Oxford Scientific, Photolibrary
Though still among the world's rarest creatures, a horse and a frog featured in the new WCS report are considered success stories.
The Przewalski's horse (pictured on a private reserve in the Ukraine) has rebounded after being declared extinct in the wild in 1966. The only living species of wild horse, the Przewalski's horse is native to the steppes of central Asia. A reintroduction effort that began in the 1990s has led to a thriving population of at least 300 horses in Mongolia, according to the WCS.
Thanks in part to a captive breeding program, the Romer's tree frog (not pictured) of the islands of Hong Kong has also shown signs of recovery.