Call of the Rabbit: Modern Communism(那年那兔那些事): Because Tu Gong (土共，Rural Communists) can be interpreted as a pun of Tu(兔), rabbit. This popular Chinese webcomic depicts China as a rabbit and all other countries as more traditional animal stereotypes. Suprisingly, Useful Notes/Taiwan is depicted as a Man.
Coney Dewclaw might as well be the Trope Codifier. Nobody who doesn't know her beforehand would be able to tell via looks alone that her mother is not only a wolf, but a top-notch hunter. She definitely got her appetite from the same, having left a long trail of the blood and bones of anybody who has attempted to prey on her, her family and her friends in her wake. The picture at the top might as well be a real-life image of what she is capable of. She has directly saved her siblings, her father, her grandmothers, her grandfather, the entire Rabbit Council, R.L., and many others in separate incidents via her skills/appetite.
Her dad, Kevin, might count as well. He's three times the size of any other rabbit we've seen in the strip (including his parents and sister) and mostly muscle (in fact, he used to have a side job as a masked wrestler).
Bun-Bun. He once skeletonized a shark that had the misfortune of attacking him, and went on a holiday-massacring rampage as part of his vendetta against Santa Claus.
He has recently been revealed to have once operated as a Physical God.
Also in Sluggy Freelance there's "The Evil", 18 (6 + 6 + 6) "demon kittens" that go on a killing spree if they have not had milk in the last 24 hours. Of course, these kittens are literally the spawn of Satan. They show up in two story arcs found here and here.
"Kicking a kitten... A grown man punting a kitten who was looking the other way... It was the bravest thing I've ever seen."
Kieri from Slightly Damned. When she first meets Buwaro, she changes into a bunny and then chases him around Rhea's house with a steak knife. Link.
8-Bit Theater featured the diminutive but dreaded camel spider (based on a real-life arachnid, whose threat is greatly exaggerated), which "viciously attacks anything it perceives as a threat, which is anything it can perceive."
Arguably, the entirety of Bunny is based off this trope, particularly Orange Bunny.
Slim the Jackalope from NeverNever has sixguns. Very tiny sixguns. And he bites. Extremely hard.
Schlock Mercenary features a species of "cute and cuddly koalazoids" known as the Tausennigan Ob'Enn. The Ob'Enn, despite their small size and disarming appearance, are referred to as the "psycho bears" by every other species in the galaxy, as they have one of the most powerful and advanced interstellar militaries in the galaxy, and are led by a xenocidal doctrine that calls for the destruction of every other species in the galaxy. At one point a mob boss even mockingly calls Petey — an A.I. created by and resembling the Ob'Enn — a "hairy, scary drop bear."
Lepus, the awakened Fiendish rabbit from Dungeon Crawl, inc. As the page's title suggests, he was originally a homage to the original Killer Rabbit (before he became a full-fledged character).
Reynardine, in Gunnerkrigg Court. He's apparently a cute little stuffed toy. He's actually a fearsome possessor demon that kills the body of whoever he takes over. He just happens to be stuck in Antimony's toy. For now. An illustrative example.
The Racconans. They're cute, fuzzy two-foot tall talking raccoons... who turn into lightning-flinging shredding whirling balls of death when backed into a corner. A typical saying in the Questorverse is "there's no such thing as an unarmed Racconan."
Quentin: You know, Groonch, there is an old saying in the seven systems about an unarmed Racoonan... pity you never heard it.
Alternatively, the Redcaps, who bear a marked resemblance to the little dancing mushrooms from Fantasia, till the needle-sharp claws and fangs come out and they leap into your face. Those bright red "mushroom caps" they wear are actually BLOOD SACS, and they've been known to drain an ox dry of blood in moments...
The Video Games section paid plenty of attention to Gaia Online's MMO aspect, but Grunny from the webcomic has yet to be mentioned. It is a predatory green rabbit that also carries the G-Virus and turns its victims into zombies.
Dolphins in The Adventures of Dr. McNinja. You could punch out and kill all the gorillas, dinosaurs, giant lumberjacks, ninja, vampires, and ghosts you want, but if you treasure your continued existence, leave the dolphins alone.
Digger and other wombats may look small and chubby, but they spend most of their lives swinging heavy pickaxes, and are, as she once put it, "like biceps with feet." When pressed into battle, Digger has shown herself to be an effective combatant, not only soundly defeating a gang of bandits, but also lecturing them on what poor condition their weapons are in.
Mr. Boodles, an average-looking cat from Eerie Cuties, proved to be this in the November 18th update. He is also the pet of a vampire queen.
Erfworld's Transilvitoan bats are among the weakest units in the world (generally only useful as scouts). However, when boosted by the right leadership bonus (and with sheer weight of numbers), they become capable of taking on boosted dwagons.
And from the back of the print version of book one: One world's "cuddly" can be another world's "deadly."
Swedish online RPG satire Rubicon survival guide use a killer hamster to demonstrate the dangers of taking the rules too literally:
Horrified Barbarian: A HAMSTER!!! OH MY GOD!!! Confused Wizard: What???? Horrified Barbarian: A hamster bite is one damage point, and it bites and bites and bites! It can take hours before you die! A slow, agonizing death ... LOOK OUT, IT'S ATTACKING!!!
Subverted. The Beemahs, despite their natural energy weapons, lack the organ infrastructure for survival and are wholly dependent on others. They needed help to defeat their creators, and their helper promptly enslaved them himself... as well as placing a kill-on-sight bounty on them so they wouldn't try to turn onhim.
The paedagogusi of Errant Story. They're fairy-type people about the length of a hand. They're also no one to mess with.