First edition cats had higher damage bites (1-2) and automatic "rake" attacks - if the cat hits with its combined claw attack for 1 dmg, it automatically hits with both rear claws for 1-2 more damage. Five damage will severely injure or kill most first level first edition PCs - the absolute maximum being fifteen for the hardiest dwarven fighter with maximum HP. As for your first level mage, five damage will flatline 90% of them - only max hp/max con bonus allowing survival till the next round.
The original "Arduin Grimoire", a very early third-party supplement, included among its bestiary a creature called the "killkitten". Resembling normal kittens, these beasts were actually cunning pack predators whose hollow claws could inject a paralytic poison. Their normal procedure was to set one of their number as bait to attract a potential victim by acting like an injured kitten, while the remainder of the pack lurked, hidden, nearby. When the unsuspecting schmuck picked the "kitten" up, it would paralyze him with its venom, after which the rest of the pack would swarm and eat him.
The Tibbit, a player race introduced in Dragon Magazine, are perfectly normal humanoids that happen to be able to take on housecat form at will. One of the illustrations on their section in the Dragon Compendium is a man lying facedown in a pool of blood, with a metric ton of cutlery jutting from his back and a glowering kitty crouched on top of him.
Kobolds, having low HP and a reputation for cowardice, are frequently slaughtered in the open by first-level parties. They're also known for their trap-designing prowess, meaning that a clever DM can make a trap-filled death maze that can frighten well-prepared parties. The most popular recount of this happening is "Tucker's Kobolds", named after a particularly nefarious DM. And then there's Pun-Pun, a kobold wizard build that serves as the ultimate Game Breaker.
The 3.5E Monster Manual IV introduced the Skiurid, an evil squirrel from the Plane of Shadow, and generally regarded as one of the worst monsters D&D ever introduced. Then an infamous column on the Wizards of the CoastD&D section came along, specifically looking for ways to make Skiurids lethal. The squirrels are quite capable of bringing even a mid-level party of well-equipped adventurers to their knees.
The giant shrew, a critter from Basic D&D, looked like a normal-sized grayish rat, yet could do a pretty good Vorpal Bunny impression on low-level adventurers.
Quite a few fey arguably qualify — the well-known nymph, for example, has one ability you don't hear about too often: if you happen to catch a sight of the nymph naked, she can force you to make a fortitude save or die on the spot. Other kinds of fey are more "harmlessly cute" than the nymph, but tend to have a wide range of powerful magical abilities. Very few fey are harmless. In fact plenty of them will do horrible things to you given the chance.
Then there's the wolf-in-sheep's-clothing, a rival with the skiurid for one of D&D's worst monsters. It's not the bunny you need to beware, so much as the stump it's sitting on...
"Blink bunny", called Al-mi'raj ("experiment 72") by the gnomes of Krynn. It's just a stupid bunny, but it can teleport around, is nearly fearless and has a unicorn-like horn which it doesn't hesitate to use if threatened. Oh, and it's a herd animal. Now the bad news: one of ten adults have Psychic Powers with a ridiculously large power pool. The dreaded rust monster may decay one's sword or armor if it manages to hit it with its antennae, but a psionic blink bunny can blast it into a cloud of shrapnel just by looking at it. It also can set stuff afire and control flames so that it or its nest isn't hurt. While flying around. It first appeared in 1st/2nd edition Dungeons & Dragons in the original Fiend Folio, and is based on the Miraj (see Literature). The very first version of the Al-mi'raj was just an ordinary, very fast, dimwitted, unpredictable rabbit that appeared in packs of 2-20 and which could deal damage as per a dagger by stabbing someone with their horn.
The 3.5 version (appearing in the OGL-based "Tome of Horrors" books) underwent a severe cutback, being only CR 1/2. This version of the Al-mi'raj sits between the 1e and 2e versions, being capable only of teleporting (Blink as a free action, Dimension Door 1/round) and stabbing with its horn like a weak dagger (1d4-2 damage).
The Pathfinder version, though, went back to being a true Killer Rabbit. Though it lost its teleporting powers, it became a whirling, acrobatic, carnivorous dervish of death that cripples prey by stabbing out their legs and then eating them alive — a necessity, as its horn petrifies creatures that it slays.
Brain moles, which absorb energy from psions to turn them into life, and which spread a psionic disease. They're not hugely lethal, but their challenge rating is still about on par with that of a level one warrior.
Hackmaster 4th edition
The Killer Bunny is featured as a random encounter in this game. This creature is extremely dangerous, doing 100 Damage every hit. The cute looking rabbit is known to cause a Total Party Kill. Any adventurer that has ever survived such a random encounter will run for their lives or at least be very cautious whenever they unexpectedly encounter a cute bunny.
Deadlands: The Weird West features jackalopes, which are evil and carnivorous versions of the infamous "rabbits with antlers" from Wild West folklore. They kill their prey by cursing them with bad luck, then stalking them until they suffer a fatal accident. It also has "Dusters", which resemble dust-covered, scrawny versions of small harmless mammals — squirrels, cats, rabbits, whatever — who can evaporate all water around themselves. But killing people by drying up all their water in the desert isn't enough for them. See, if a Duster can touch a living creature — like, say, a human — it can suck all the water out of that creature, mummifying them alive...
GURPS IOU includes lethal versions of rats and squirrels.
Cockatrice is a turkey-sized mix of lizard and pheasant, looking mostly like the latter. Doesn't sound intimidating? It can easily sneak up and peck out kidney or spine of their victim with with a single strike. One success more than required during the attack roll, and it will reach a vital organ with its peck, dealing an additional 3d6 damage and another d3 from bleeding each round. That's enough to kill or incapacitate a character. To make matters worse, it has enough Sneaking skill to easily ambush a character, thus making it even easier to strike a killing blow.
Echinopses are almost like porcupines. The only difference is that they can shoot a few of their spines for a very short distance. If those spines hit anything, they break and migrate deeper. It's not only painful, but also requires an operation or strong magic healing to remove them. Untreated, such wound will eventually kill a character.
The setting, known for a habit of Everything Trying to Kill You cropping up everywhere, brings us the Catachan Barking Toad: a large, sad-looking amphibian sometimes dubbed the "Ronery Toad". If attacked, hurt or even surprised, it explodes into a cloud of obscenely virulent toxins, killing absolutely everything for miles around and poisoning the earth so that nothing will ever grow there again.
The original "Rogue Trader" book for Warhammer 40K also featured the "Catachan Face Eater" a carnivorous creature that looked quite a bit like an ordinary wash cloth.
Wastits in HoL — small, cute waddling soft creatures resembling animated teddy bears that will suddenly "explode into a maw of teeth the diameter of a whale's privates". For added fun, until they attack they're almost indistinguishable from wastems, completely harmless creatures that are the primary food source of HoL's inhabitants.
Most of the creatures that Shadowrun's shapeshifters start out as are pretty dangerous on their own (lions, bears, eagles, etc.), but a fox magician character can lead to a cute little ten-pound fox hurling fireballs.
Due to it's mechanics (open ended attack rolls and instant death criticals), any creature in Role Master could fit this trope. Your level twenty badass fully armoured dwarf warrior can get his throat ripped out by a single rat with a lucky set of dice rolls.
Urseminites in Bulldogs are essentially Ewoks with an extra-bad attitude, looking like living teddy bears with not even much in the way of natural weaponry but having a deserved reputation as little psychos who, to quote their description, "revel in vice". There's a not implausible in-universe theory that they're the result of an experiment in domestic genetic engineering gone horribly wrong...and they can be player characters.
Zot the Wizard from Red Dragon Inn has a literal killer rabbit in the form of his familiar Pooky. A later expansion even makes Pooky playable as a stand-alone character.
Gimcrak the quasit from the PathfinderWrath of the Righteous adventure path. He's an Ugly Cute Ken doll-sized demon in a tiny leather BDSM outfit. Why's he wearing all that black leather? Because he has 10 levels in assassin, and he is scarily good with his tiny little knife.