They're cute. They're cuddly. But beware: they're killers.

1. KOALAS

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It may seem hard to believe, but the world's most cuddly animal has a dark side. Most of the time, these tree-huggers keep to themselves, adhering to a strict schedule of snacking and snoozing (up to 22 hours a day). But sometimes, a koala snaps. Koala-on-koala violence is generally pretty mild, but they have been known to go after dogs and even humans.

For example: In December 2014, Mary Anne Forster of South Australia found herself at the receiving end of a vicious bite after trying to protect her two dogs from an aggressive koala. The koala sank its teeth into Forster's leg and refused to let go, relenting only after she reached into its mouth and pried its jaws apart with her hands. Forster then walked her dogs more than a mile back to her house before going to the hospital for stitches, proving that the only thing tougher than Australian wildlife is an Australian.

2. BEAVERS

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They've got huge, razor-sharp teeth that never stop growing. They're fiercely territorial. They build complex underwater lodges with architectural precision. And, most importantly, they don't take crap from anybody. There was the fisherman in Belarus who died when a beaver bit through his femoral artery. There was the lake in an Alaskan dog park where angry beavers sent a half-dozen dogs to the emergency vet for stitches, prompting park officials to post signs reading "WARNING AGGRESSIVE BEAVERS ARE LIVING IN UNIVERSITY LAKE!"

And those are just the healthy, well-adjusted ones. Rabid beavers have gone after swimmers in Canada and the U.S., including an 83-year-old woman in Lake Barcroft, Virginia. "There is no way I will swim in that place again," she said after the incident.

3. COWS

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Not all cows. But some cows. And those cows are terrifying. They're called Heck cattle, also known—and we are not making this up—as "Nazi Super Cows."

In the 1920s and '30s, German zoologists (and brothers) Heinz and Lutz Heck each sought to recreate the extinct wild ox called the aurochs, which featured heavily in Teutonic mythology. Heinz Heck chose Spanish fighting cattle as a breeding strain for their prehistoric shape and aggression, and the Nazis used their fierce image in propaganda. Then, World War II happened. The Nazis fell, but the uber-cows survived.

Heck cattle still roam Bavaria to this day and are available for purchase by those with a death wish. Farmer and photographer Derek Gow brought a herd of Heck cattle to his UK farm in 2009 and successfully bred them before realizing he was in over his head. "They would try to kill anyone," he told The Guardian of the dozen or so he had to put down because of the danger they posed. "Dealing with that was not a lot of fun at all."

4. DOLPHINS

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It sounds outrageous, but it's true: dolphins are actually pretty horrible.

Researchers have suspected as much since the 1990s, when the battered corpses of hundreds of porpoises and baby dolphins started washing up on both sides of the Atlantic. Eventually the researchers concluded that male dolphins were slaughtering other dolphins, including their own babies, just because they could [PDF].

This news was especially alarming to federal officials, who were concerned about human safety in the growing and unregulated industry of dolphin tourism. "It's a time bomb waiting to go off," said a spokeswoman for the National Marine Fisheries Service.

If that wasn't bad enough, dolphins have sexually assaulted divers and swimmers on numerous occasions, and have been known to play volleyball with helpless baby sharks.

5. PRAIRIE DOGS

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Move over, dolphins: you’ve got competition for Most Horrifying Murder Spree. And prairie dogs, as it turns out, do not take kindly to competition. Researchers say white-tailed prairie dogs routinely hunt and slaughter ground squirrels, with which they compete for resources.

The prairie dogs are plant-eaters, so once they've bitten the squirrels to death, they just drop the carcasses and stroll away. The first time prairie dog expert John Hoogland saw it happen, he was shocked. "It boggles the imagination that something like that was going on under our noses and we didn't notice," he told New Scientist (which—be warned—includes Hoogland's gruesome images of the carnage in its story).

Unlike the murder-happy dolphins, however, the prairie dogs have a clear motivation. Prairie dog serial killers (that is, those that just kept killing) tend to have more babies than non-killers, and they and their offspring are more likely to survive.

"It begs the question of whether it's going on in other species," Hoogland said.

6. SLOW LORISES

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After the dolphin, the slow loris looks like a saint. A shy, weird saint, with a mouth full of needle-like teeth and venomous elbows.

Yes, the slow loris has venomous elbows. When a loris feels threatened, it throws its arms over its head. This is adorable, but it's also strategic, giving the little primate an opportunity to lick the toxin-producing glands in its upper arms and fill its mouth with venom. While the venom itself is only strong enough to kill smaller animals, loris bites have sent humans—including one researcher—to the hospital in anaphylactic shock.

Some scientists argue that the loris's elbow grease isn't venom at all, and that its ability to kill is purely incidental. But … this is probably not much comfort to someone who's just been bitten.

7. SWANS

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Like most cows, most swans are fine. Sure, they get a bit territorial during breeding season, but who doesn't? But the swans that are not fine are really, really not fine.

Take Hannibal, the swan who killed 15 other swans and injured dozens more on the grounds of Pembroke Castle in Wales. Hannibal bit his victims, beat them with his wings, broke their toes, and held their heads underwater until they drowned. After each brutal attack, Hannibal would parade in front of his kill, displaying the carnage for his wife—Mrs. Hannibal—and cygnet.

And then there's Mr. Asbo, the swan that terrorized rowers on the River Cam in Cambridge for years. Mr. Asbo (short for "Anti-Social Behaviour Orders") regularly attacked and even capsized small boats before turning his aggression on larger vessels. Eventually, even the Royal Society for the Prevention of Cruelty to Animals (RSPCA) agreed that Mr. Asbo was "out of control" and got a license to relocate him and his mate to another county. One year later, a young male swan appeared in the same spot and started threatening people. Locals named the cocky newcomer Asboy, after his father.

8. HIPPOPOTAMUSES

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Each year, the humble hippopotamus kills more people than lions, tigers, or bears. Or sharks, for that matter. (In the hippo's defense, humans kill quite a lot of hippopotamuses. This is not cool.) They're intensely aggressive, which is a dangerous quality in an animal that can reach 17 feet long and 10,000 pounds. They're not slow, either: They can reach speeds of up to 30 miles per hour on land, outpacing even Olympian Usain Bolt. They go after each other, after humans, after crocodiles, and even after boats and cars, flipping the crafts and attacking the inhabitants. From time to time, someone will try to tame a hippo and keep it as a pet. This does not end well. Do not try this.

This story first ran in 2015.