I think a honey badger could beat a human if it cared to.
In Fredric Brown’s “Arena,” a naked human… well, picks up a rock and kills the alien enemy. But only by employing a trick.
Well, that’s what we are good at…using tools and employing tricks to keep deadly aliens in check! It would work on our various furry animal adversaries as well. It’s why the OP really doesn’t make much sense, especially his use of ‘natural’.
I’ve had good results with mosquitoes and cockroaches. So far…
I’d say this pretty much sums it up.
Assuming a will to fight. Many people faced with a dog/wolf will go completely defensive.
Of course a single wolf would be extremely unlikely to engage an adult human, because animals generally only do so if the odds are well in their favor or theyre desperately defending young, even then, usually only as much as necessary.
I can’t think of anything much over 100lbs that would be an exception but I’m sure there’s some docile, slow critter that fits the bill if we looked hard enough.
Ironically, mosquitoes have probably killed more humans than any other complex organism on the planet…including ourselves. 
Lol
Human beings are pretty good “generalists” and more capable than one might think. First and foremost, we have excellent analytical skills. While we lack the instinctual fighting skills of many animals, we’re quite good at planning our defense/attack. Instinct is fast, but often inflexible. Second, we have resources (stereoscopic vision with excellent depth perception, opposable thumbs, decent reach, good climbing abilities, excellent stride, etc.) that can be leveraged in an encounter to help compensate for our more gracile bodies and less-than-impressive teeth.
A dog will bite you, but it won’t normally jump out of a tree on your back. A snake won’t pick you up and smash you on the ground. A cat won’t swing you into a tree trunk. A badger won’t pick up a rock and swing at your head.
Certainly we are…which is why we use tools. And hunt in groups and use strategy. Without tools a human is basically helpless. Completely and utterly. Hell, you wouldn’t have to have the furry friends kill said human…the environment would do it. And this isn’t just lack of survival skill sets of modern humans, it’s a fundamental issue. It’s why you don’t see even the most primitive tribes without tools. Basically, we haven’t been without some sort of tool for millions of years…since before we even WERE ‘human’. Like I said, it’s like asking what a lion could kill without teeth or claws…probably even beyond that, as the lion still has it’s strength and speed as well as coat. Take it’s strength, speed, balance and coat away and what could a lion kill? Very little. Take away tools and there isn’t a natural human to attempt to ‘naturally’ kill very much. How do you catch the stuff that moves fast? How do you run something down in the long haul (which we are very good at) without tools to carry water or protect us from the elements while we are doing it? How do you take on that badger without something to hurt it or catch that small cat…or kill the larger one? The answer is you can’t…no human could, reliably, even going back 10’s or 100’s of thousands of years. Especially if you are coupling the ridiculous crippling of no tools with no other humans. It is basically taking everything from us, just like taking the lions claws, teeth, strength, speed and coat away and asking what it could kill.
A sloth? They’re pretty big but so dozy and slow-moving that they probably wouldn’t put up much of a fight. Not sure how you would actually kill one though. Drown it? Jump on it? Keep poking it to wake it up until it eventually dies of sleep deprivation?
Maybe you could kill an anteater if you killed all the ants around it so it died of hunger.
Do shoes count as tools? Does clothing count? Repeated kicks combined with fingers to the eyes could give our biped a fighting chance against a variety of critters. Not that I’d want to be in such a situation.
I read the OP as a forced encounter, a “fight” between a human and something else. Not fighting the hypothetical, so we need not concern ourselves with whether the critter would prefer to run away rather than engage. Suffice it to say that, for some reason, the fight is on!
Cats of all sizes are problematic because they have 5 separate and very effective weapons (paws and mouth). I have lots of personal experience handling smaller felids (up to cougar) and I’ll tell you that a bobcat will fuck you up. You might be able to choke one out, but you will sustain significant, perhaps life threatening damage. Smaller cats (e.g., serval, etc.) may be slightly less problematic. Larger felids, no way, it would be impossible unless the animal has major physical handicaps.
Canids can probably be successfully taken on at larger sizes since they can only really damage you by biting. So you’ve got a fair chance with a wolf, even at 100 pounds, as long as you can avoid getting knocked down and your throat bitten out in the first rush. Grab ahold, hang on, and keep choking for as long as it takes.
Primates are problematic because most have enlarged canines, and are pound for pound much stronger than we are. I’m sure a person could fairly easily overcome a squirrel monkey. I have handled, and wouldn’t want to have a “fight” with, say, a pig tailed macaque because I’m pretty certain I’d lose. Hell, it might rip off my arm and beat me with it. If we go up to something like a bonobo, then no way, it would be suicide. It would require a blotter and tweezers to gather enough of your remains for a burial.
OP, does this help address your question?
I’m confident I could take down a trout.
I bet I could lick a toad as well.
There isn’t a hamster alive that I couldn’t take down in an hour or so.
That’s a good point. I think that pound for pound canids are the species that we could most take on. Even herd animals, in a fight to the death, have kicks that would be more powerful than a canid’s bite, as long as you keep the bite away from the neck area and don’t care if you are damaged. A bite might cause some bleeding but a kick can break ribs. (In the natural environment, however, we could take on a herd animal by simply making it run away until exhausted, as long as we could track it.)
However,
I think the average person could still take on a bobcat. Even if we die later of our wounds, I still think we’d count as having “beaten it in a fight”.
Boy, I wouldn’t want to be the guy who has to try to defeat a 150-pound canine in a fight. Dogs are ludicrously strong. The idea of choking them out sounds great but a dog that approaches your weight will be much too strong to not twist out of your grip. There has to be some weaker animal, surely.
Alligators are famously weak at opening their mouths once held closed. I’d rather tangle with a 150-pound crocodilian than an angry wolfhound.
Yeah, I don’t think a large dog would be a lot of fun to tangle with naked and in a fight to the death. Assuming the dog doesn’t run away, that would be nasty. Now, I’d take on a chihuahua in a cuddle to the death any day, and would die happy. Not sure I’d want to tangle with an alligator though either, naked with no tools other than my bare hands.
I suppose if we take win as a poster above is looking at it, i.e. you just have to survive the fight and the other creature doesn’t, then you can die and you have ‘won’, then that opens up quite a bit. Personally, if it’s me I’d kind of like to survive in the medium and long term as well, but I suppose if you had a committed (and I use that word judiciously) enough human who is willing to go all out to win on those terms then they could probably take a dog or your alligator or the bobcat mentioned above…then die a few hours or a day or two later.
Ludovic, there’s no way to hold onto a felid that will prevent it from raking you repeatedly, swiftly, and deeply with it’s claws. You can straddle it, trying to control the head with your hands/arms so it doesn’t bite your throat (the commonest ‘kill shot’ most felids employ on large prey) but it will then rake your legs with its hind feet. Meanwhile the front legs will be reaching for your chest and face. Ask anyone in a vet’s office how hard this is with a housecat! They use tools, not the naked human scenario of the OP. Once you get up to cats in the 30+ pound range like a bobcat, those hind claws can be digging deep enough to impact the femoral artery (barely beneath the surface behind your knee). And the front legs are long enough to reach at least your abdomen, and open it up, perhaps even your neck. As I said, life threatening injuries, and sooner rather than later. You have no equivalent way to damage/kill the cat. You can’t rip it or bite it open. You can’t hit it hard enough to cause cervical dislocation or massive concussion. It’s not just standing there while you punch it repeatedly in the head. So your only realistic chance of killing it is to choke it out - which leaves you open to the injuries noted above.
RickJay, I don’t wish to minimize the difficulty of tackling a 100+ pound canid! But other than general strength, it only has one dangerous part - the jaws. If (a big if, admittedly) one can avoid the first bite, get close and grab the head and straddle the animal, then the chokeout becomes at least remotely possible. Again, as with the bobcat, what else can one do to kill it barehanded? I’m not envisioning our human as a 90 pound weakling, nor the giant David slew, but rather at least someone in between. Someone with a [grin] fighting chance [/grin].
As for alligators (and crocodilians in general), these are within my professional expertise. I’ve caught literally hundreds of them in the wild (for various research projects) ranging from 7” hatchlings to 12+ footers. It’s possible to catch and overcome them by the simple expedient of jumping on them, controlling the head, and flipping them onto their back. Of course this requires a bit of technique, and it surely helps to have practice on smaller individuals. And the practical limit is around 7 to 8 feet. Beyond that size they’re just too powerful and wiggly so tools, ropes at least, are needed. The animal is just too strong to be overpowered and manipulated by one person. Once on it’s back, one person can probably keep it there indefinitely. Does this count as a win for purposes of this hypothetical? But crocodilians are extremely hard to kill, and frankly I think it would be impossible without a weapon. You can’t choke them, and you can’t hit them hard enough to cause even minor damage. Their skulls are massive, their brain tiny; your most emphatic stomping will have zero effect.
https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.dailymail.co.uk/news/article-1338906/amp/Jogger-kills-savage-dog-bare-hands-attacked-son-7.html&ved=2ahUKEwioseOVv_3fAhVRMt8KHduwC_UQFjAFegQIARAB&usg=AOvVaw2y_MGFffXlvN4VEiMdhbQw&cf=1
https://www.google.com/url?sa=t&source=web&rct=j&url=https://sputniknews.com/amp/world/20130905183180312-Kazakh-Man-Kills-Wolf-with-Bare-Hands--Report/&ved=2ahUKEwjGjYOsv_3fAhUCnOAKHXFcBWoQFjAFegQIAhAB&usg=AOvVaw0XTcgcx9JTyYw8Ryxzp31L&cf=1
Bobcat, large dog, wolf. Leopard is also easily found. Large dogs are killed with some regularity by barehanded people. Multiple bobcats and leopards. I’ve only seen the one wolf.