Far, far too many individual examples to list, but chances are that a good percentage of mammalian, reptilian, and avian predators (in their infancy, at least) are strong candidates. Realize that, cuteness aside, any animal that hasn't lived a completely domesticated life has to survive in nature, fighting for food (not to mention having to kill it), protecting its young, and every once in a while, beating the hell out of something that wants to eat it. Cracked presents the six cutest animals that can (and will) destroy you.
Cats ARE Badass Adorable. Humans just notice the adorable part more often than the badass part.
Some articles have described cats as not being truly domesticated. This makes sense. If your cat leaves home, he or she has a great chance of being able to survive in the wild hunting things like a furry little Rambo. Your chihuahua, not so much. note Cats are domesticated, but not nearly on the same level of dogs. Unlike with dogs and livestock animals, humans never actually made an active effort to domesticate cats. When agriculture was developed, humans were able to store food for long periods of times. This attracted rodents like mice and rats. The cats learned that where there were humans, there were mice, so cats came by to eat and only tolerated the humans. Humans only tolerated the cats because cats killed rodents that otherwise would've wiped out the food supply.
Sand cats, a particular variety of small cats found in the desert, and one that looks much like domestic cats. Just look at them!◊ How could that not give off both a "badass" vibe and an "adorable" vibe at the same time?
There's a youtube video of a cat playing with a kid when suddenly an alligator shows up... and the cat scares him off. What does the gator do? He comes back with a buddy. What does the cat do? He swipes at both of them.
This kitten. A video titled "Brave Kitten Stands Up to Dog". The kitten is tiny and cute, and the dog is really huge. More to the point, the kitten charges the dog at least once.
Domesticated cats kill more types of animals than any other cat species, even the biggest ones like lions and tigers. Domestic cats are very good at killing scorpions, to the point that, in places where scorpions are common, cats are kept around to kill them.
Cats have been known to chase away BEARS.
This bulletin highlights the badass part even more. 
Jumping Spiders. They're tiny, fuzzy spiders with big eyes that are universally considered adorable, especially amongst spiders in general. They can also jump distances of 20 times their size, are amongst the most intelligent of arachnids, and can catch fast moving flies in midair, or even other spiders.
Dogs. They are adorable with their big eyes, ears, droopy/smiling faces and wagging tails aren't they? Well, some have been bred for herding livestock, hunting, rodent control, guarding, helping fishermen with nets, and pulling loads, in addition to their roles as companions. They can also serve as service dogs such as guide dogs, utility dogs, assistance dogs, hearing dogs, rescue dogs, and psychological therapy dogs provide assistance to individuals with physical or mental disabilities. Not to mention there are some pretty capable police dogs out there who sniff for bombs/drugs, chase after suspects, and can sniff out decomposing corpses. They are the descendants of wolves, who are known to have incredibly powerful, bone-crushing jaws, ridiculous amounts of stamina, and hearing and sense of smell beyond human comprehension, and enough intelligence to work together and use surprisingly complex strategies to take down prey several times more powerful than themselves. Almost all dogs have at least some of these traits left over. Suffice it to say that most domestic dogs, including the smaller ones, (barring such intense human manipulation as is seen in the chihuahua) are almost as tough as their wolf ancestors- and a few are even tougher. Dogs are wolves, or rather a sub-species of them. Even the most tame and lovable puppy dog has those instincts running around somewhere, waiting for the right trigger to bring it out. Which is why so many of them love to play fetch. They have been carefully bred for thousands of years to be as domestic as possible, but almost everyone has seen a loveable dog go absolutely Papa Wolf when he thought his family was being threatened. Recognition of this trait is an important part of responsible dog ownership. See also thesearticles from Cracked. For a specific example, see the Dachshund. Yes, they look a bit like a sausage with short, stubby legs. Then you realize that the reason they look like that is because they were bred to fight badgers.
Hummingbirds. Tiny little balls of feathers and hyperactivity that live off sugar water and look like digital camera ads. They're also stupidly aggressive, and will chase birds ten times their size out of their territory. There's a reason why the Aztecs believed hummingbirds to be the reincarnations of their own warriors.
Chimpanzees and other apes can be pretty cute at times. They are also possess bone-crushing strength and incredible ferocity when threatened. Also, unlike most animals, they're smart enough to remember if someone's previously done something to make them angry. And to plan out how to get back at that person. A zoo chimpanzee once collected a bunch of rocks in his enclosure and then stocked them for later. Once visitors began to annoy him, he started flinging the rocks at them. Fortunately, his aim wasn't very good, so no one was hit.
Skunks are quite adorable if left alone. However, they tend to walk around rather boldly and have been known to stand up to bears and mountain lions, normally ending by sending the predator running with a face full of musk, and if they're smart they don't come back. If their spray fails, they can resort to tooth and claw fighting, and can seriously mess you up. And mother skunks are quite protective of their children, making them even more Bad Ass.
Painted wolves base their pack hierarchies around submission, resulting in puppy-like antics from just about the entire pack. What makes them noteworthy separate from other canines? 90% of their hunts end in kills - even lions only manage a measly 30%. And consider how they kill their victims. Does the term "live disembowlment" mean anything to you? Beware the Nice Ones.
Leopard geckos◊, more well-known for their easygoing nature as a pet, have a secret talent: they can kill scorpions larger than themselves.
Mongooses have this reputation for their odd ability to defend themselves against venomous elapid snakes, mainly cobras. However, this seems to be semi-based on Reptiles Are Abhorrent attitudes, so it's a bit of a good-bad reputation.
Velvet worms. Probably another Ugly Cute example, but they're soft and squishy things that crawl slowly over the forest floor on stubby legs. And are predators. That can kill prey the size of tarantulas. (They do it by spraying a sticky slime that quickly hardens into a net to trap their prey. And then they close in and eat the helpless prey alive.)
Tasmanian devils. They're pudgy, fuzzy little black critters with a perma-scowl who are infamous for their frenzied screeching and the sheer ferocity that they display when eating, not to mention having the strongest proportional bite of any living mammal.
Smaller birds of prey, like falcons and small hawks. The Peregrine Falcon is the fastest living thing on Earth, and dive-bombs aerial prey, killing them in mid-air. It will use this same technique to chase off other raptors several times its size.
Sea turtles, the Iron Woobies of the ocean, are among the few animals to be resistant to the poison of the Box Jellyfish, the most venomous creature in the world, which they regularly eat.
Badgers in general. They have cute little stripey-faces, stocky bodies, and also have razor-sharp claws and teeth which they can use to defend themselves (Though, normally, they will just try to burrow away. Those claws are mainly for digging, after all). Special mention goes to the Honey Badger, which has become a Memetic Badass due to its fearless nature.
Grasshopper mice. Grasshopper mice are Territorial, aggressive, and one of the few carnivorous mice. You read that right, these little guys hunt grasshoppers, worms, spiders, tarantulas, scorpions, and even other mice. They even howl at the moon to mark their territory and scare their rivals. Basically, they're wolves, in mouse form.
A lot of smaller predatory dinosaurs sometimes get this portrayal, particularly Velociraptor and its relatives, in light of the overwhelming evidence that they had feathers.
Dolphins. Don't let their cute appearances and playful antics fool you, they are actually capable of killing sharks. Just to put you in the right mindset; You know how they're always smiling? They aren't smiling because they're that happy all the time, they're smiling because that's just how their faces look.Sound familiar? Not only are dolphins the only other known mammal, aside from humans, that will copulate for pleasure as well as breeding, but they're also the only other known thrill-killer in nature aside from the domestic housecat...
The orca in particular isn't called killer whale for no reason. Even great white sharks seem to be afraid of them. Also remember that the name is backwards. They were originally known as "whale killers". WHALE KILLERS. Their scientific name is Orcinius Orca. Orca means a Kind of whale. Orcinius means Realm of the dead. Their Scientific name is Whale of Hell. And HOW they kill whales? They torpedo themselves into their stomach.
Bearded Dragons. One of Australia's many lizard species, they're about 9 inches to a foot long, with a stubbly "beard" of soft bristles under their chin. Totally harmless, and indeed a popular pet with local lizard-fanciers. These things will eat Whistling Spiders — the Australian equivalent of Tarantulas. Fangs and all.
You know Tyrannosaurus rex? Well, according to discoveries in paleontology, it was covered in feathers. Not Bad Ass flight feathers like an eagle, but fluffy, downy feathers like a baby chicken. And remember how terrifying the T. rex was in Jurassic Park? Now imagine that with fluffy baby feathers. D'awww
Budgies, usually thought of as silly little cage-potatoes that whistle when you say "Who's a pretty boy, then?", are some of the most agile small birds on Earth. In the Australian outback, flocks of budgies regularly out-maneuver falcons at watering holes, so easily and confidently that they'll even stop to grab a quick drink ''in mid-chase''.
Flamingos. Oh sure, they look dainty and fragile but think about where they live; they don't live in big pretty blue lagoons like the media tells you, they much more commonly live in boiling crater lakes that are so hot and full of saline that very few other animals could ever survive in them. There's a reason the Ancient Egyptians thought these birds were phoenixes.
Squirrels, due to their sheer determination when attemting to raid bird feeders.
Bears. Most of them just look like big, fluffy dogs (which is appropriate as canines are their closest relatives besides each other of course.) Seeing them in zoos can desensitize people to them especially polar bears and panda bears. The former are the most carnivorous bears by necessity and the latter are often underestimated but are all too willing to protect their young with fang and claw.
Koalas, which are not bears, are fuzzy little creatures that spend most of their lives napping, clinging to trees and eating leaves with easily the nutritional value of a small cardboard box (hence the napping - they don't get enough energy from their diet to be really active creatures for more than a short period every day). At the same time, however, when surprised or aggravated they briefly transform into a vicious flurry of claws while making a noise like demonic possession in action.
In New Zealand lives a parrot known as the Kea, or Nestor notabilis in Latin. And notable it is. At first glance, it just looks like a big green parrot◊. A very intelligent and curious bird colloquially known as "the clown of the mountains", keas are popular with tourists for their silly antics, though they can sometimes fly away with people's possessions. Keas are able to create and use tools and solve logical puzzles. But what they're mostly known for is the silly little quirk they have of attacking live sheep, just tearing chunks out of their flesh.
A lot of boot camp graduates can qualify as Badass Adorable. A lot of them are only 17, 18, or 19 years old at the time, so they can look just as adorable as any other young adult. And they fight wars.
Kyra Gracie. Cute as a button, could take anyone in a grappling contest and walk away still looking perfect.
Ishikawa Rika once broke 12 concrete tiles with one blow from her bare hands, sure it turned out they were especially designed to break, but when Takahashi Ai tried to do the same thing on another show she failed miserably until she resorted to stamping on the stack instead.
Artistic gymnastics. No padding, doing amazing athletic feats that could (and has) lead to severe, career-ending injuries or permanent paralysis and even death. At the elite level, training comes first in life, up to eight hours a day, 5-6 days a week, even as a child and preteen. Many girls leave home to further their training (in the case of Chinese, Russian, and Romanian gymnasts, they live in a central training facility and rarely see their families). The effects of the pounding their bodies take day in day out leaves them with the skeleton of an osteoporotic sixty-year old. Their bodies have broken down by their early twenties, late twenties if they're lucky.note Unless they're Oksana Chusovitina, who is possibly an alien and definitely a freak of nature in the best possible way. They've likely have had multiple surgeries. The sport is synonymous with eating disorders and abusive coaches. Their only uniform is a skintight leotard. If they make it to the Olympics and are part of the four superpower teams (USA, China, Russia, and Romania), they'll be featured in front of an international TV audience of millions and have to keep cool under immense pressure, especially if they're the one who was built up to be the golden girl. They're generally between the ages of 15-18, with some exceptions.
Gina Carano◊. Cute, beautiful, seems to be on the verge of a giggle most of the time, and is a professional kick-boxer and mixed martial artist.
Audie Murphy: Just click through and look at the cute, apparently-teenaged boy wearing what appears to be his dad's, granddad's and uncle's medals, and then scroll down to learn that he earned them all himself, and the crazy things he did in the process.
This little girl who defends herself against an attacker in an elevator. She starts whooping him before the guy even touches her! And at the end, not only is he running away from her, this was his second attempt to flee, with her dragging him back inside the first time!