Mute Swan

Mute Swan

Despite their name and reputation for grace, these beauties hiss, grunt, and bark. The mute swan is one of the most aggressive waterfowl species in the world, and it becomes especially combative when nesting. According to the Forest Preserve District of Cook County, Ill., dogs and children approaching swan nests often are injured. One story even claims that a swan’s wing broke a man’s leg. 

(Photo: Andrew Steele/Getty Images)

Slow Loris

Slow Loris

When a video of a slow loris being tickled went viral in 2009, many people wanted their own tiny, bug-eyed primate. But keeping the furry animal as a pet is a terrible idea: Not only is that prohibited (an illegal pet trade has driven the threatened exotic species to near extinction), but slow lorises are the only venomous primates in the world. Toxins that they release from their elbows when threatened have been reported to cause anaphylactic shock. 

(Photo: Irawan Subingar/Getty Images)

Leopard Seal

Leopard Seal

Seals may be known as the dogs of the sea, but leopard seals are, well, the leopards of the Antarctic Ocean. The spotted marine mammal ranks next to killer whales as the Southern Hemisphere’s top predator. It uses its sharp teeth to eat penguins and other seals, but research teams that fly to Antarctica have been at risk as well. In 2003, a leopard seal killed a biologist who was exploring marine life in the water.

(Photo: Art Wolfe/Getty Images)

Puffer Fish

Puffer Fish

Scientists believe that the puffer fish, also known as the blowfish, evolved to inflate because of the clumsy way it swims. Its movement in the water leaves the fish vulnerable to predators, but swelling into a ball makes it inedible. Don’t let the cute backstory fool you: One puffer fish contains enough poison—it’s 1,200 times more toxic than cyanide—to kill 30 people. Oh, and there is no known antidote.

(Photo: Karen Doody and Jodi Jacobson /Getty Images)
 

Platypus

Platypus

When British scientist George Shaw received a platypus specimen from Australia in 1799, he was sure somebody was hoaxing him. Since then, the animal, which looks like a duck-beaver-otter combination, has endeared many to its outlandish features. Just don’t get too close: Males have venomous stingers on their feet that can wipe out small critters. Their poison doesn’t kill humans, but it can cause excruciating pain.

(Photo: Greg Wood/Getty Images)

Poison Dart Frog

Poison Dart Frog

These amphibians hang out in tropical forests, from Costa Rica to Brazil, wearing some of the most vibrant colors on Earth. Their shades can range from yellow to red to black, but don’t get distracted: A tiny golden poison dart frog has enough venom to kill 10 humans. The indigenous people of Colombia have tipped their blowgun darts for centuries with these frogs’ powerful poison, hence the name. 

(Photo: Ferdinando Valverde/Getty Images)