Grizzly bears are breeding with polar bears, leading to the existence of large, ferocious, carnivorous bears that live in forests, hikers beware.
As far as sharks are concerned, there are four species in particular that really take the title:
The infamous Great White Shark. While it does not target humans as often as movies suggest, they are so large, and have such a powerful bite that even a single "investigative bite" can be enough to sever limbs, and cause massive blood loss.
Tiger Sharks. While they aren't as large as Great Whites, they are much more aggressive, and do not share Great Whites' distaste for human flesh, as they will eat literally anything they can fit through their mouths.
Whitetip Sharks. Smaller still, and less aggressive, they are dangerous for two reasons: they also will eat nearly everything, and they swim in schools.
Bull Sharks. Possibly the most dangerous on this list. Despite their smaller size, they are extremely dangerous due to their extremely aggressive nature, fierce territoriality, and the fact that they can not only survive, but thrive in freshwater rivers.
Brown recluse spiders. They like sleeping in dark places, and if you're unfortunate enough to get bitten by one, at best, screaming agony and a long recovery process. At worst, one can lose a limb or be rendered deaf. Thankfully, for the most part they're really shy creatures. However, they tend to inhabit the same areas as Wolf Spiders, and because the two are of a similar size and shape (especially in a dark room), one can often be mistaken for the other. Wolf Spiders, while venomous, are non-lethal, fairly non-aggressive, and tend to leave people alone. Mistaking a Recluse for one can be deadly.
Speaking of spiders, there's also the infamous Black Widow. Their bite is almost painless, but it results in body pains, difficulty breathing, and rarelynote Usually only in the very old or very young, death.
The world's oceans have plenty of these (besides sharks):
Box jellyfish can easily kill a person in less than 5 minutes with one sting, they're hard to see, and there's even an almost-impossible-to-spot tiny version (Irukandji) whose sting is painless at first, but then it develops into an unbearable full-body pain to the point where the victim can only roll on the floor screaming. There's also the Man O War which just floats around where ever the currents take them, but can kill anyone unfortunate enough to be stung by their waving stingers.
The blue ringed octopus. Small, pretty looking, and its saliva has a deadly neurotoxin that's ten thousand times as powerful as cyanide and will kill you within minutes.
Cone Snails. A very pretty looking conical snail that wields a harpoon loaded with paralytic venom. Hope you can swim properly after picking it up...
Rock Fish/Stonefish. They're the only poisonous fish to worry about, they have spines in their back that are poisonous enough to kill anyone unfortunate to step on them. And it's very easy to do so, since they're really good at pretending to be rocks.
Lionfish are an invasive species, and can leave formerly thriving reefs nothing but barren rock swarming with lionfish, with a minority of anything that they haven't destroyed and coral. They're covered in venomous spines, and the venom's effect varies from "fine in an hour" to "the affected limb swells to twice its size and recovery is measured in months." If you're stung in the ocean, you will probably die from being unable to swim to safety. They will eat anything, and almost nothing will even try to attack them- save the grouper, and that's only adult grouper, since the lionfish will eat all the grouper fry. The only way to hunt them is by spearfishing, which carries insane risks (such as being stung by lionfish or attacked by any of the above).
Many prehistoric animals, like Tyrannosaurus rex, Megalodon etc, fall under this. For example, T.rex had the strongest bite force of any land animal to have ever lived, could reach lengths of up to 42 feet, and could reach speeds of up to 25 miles per hour. It took an asteriod the size of Mount Everest to finish them off. Despite this, T.rex was actually the seventh largest land predator.
Speaking of which the Megalodon was basically a great white shark the size of a whale. It's also big enough to eat whales.
There's a Demonic Spider Plant Monster in Russia and Eastern Europe, called giant hogweed (Latin Heracleum sosnowskyi, Russian borschevik). This is a plant, just as invasive as kudzu, the sap of which causes chemical burns. Yes, honest to goodness chemical burns, like mustard gas. There are many cases of child casualties caused by unknowingly messing with this plant. It also can grow to ten or fifteen feet tall, forming massive hazardous shrubbery. The worst part? It was cultivated on purpose by the Soviet authorities who thought it could be good fodder for cattle (it isn't). Hence the common nickname "Stalin's revenge".
Some Victorian prat introduced them into Britain because he thought they looked nice. Kids who don't know what they are sometimes use the nice hollow stems as blowpipes or pretend telescope tubes... honest-to-goodness chemical burns about the mouth and eyes. Heck, Genesis made a song about the fight between hogweed and humans...
The Americas have their own version of the Demonic Spider Plant, the Manchineel. The sap of the tree can inflict chemical burns, even just by being under the tree during rain. All parts of the tree are poisonous, and burning the tree produces a cloud of corrosive smoke. The Carib tribes used the sap for their poison arrows.
Cholla cactus. Okay, so it's not lethal, but it'll sure as hell stop you short if you're trying to do much of anything. It looks deceptively innocent compared to the many other horribly spiny things that inhabit the Southwestern United States and Northern Mexico; it's also called "teddy bear cholla" because it actually looks fuzzy unless you're very, very close. As it happens, the real reason for this appearance is that it's absolutely covered in semitranslucent, needle-sharp, barbed spines about as thin as a single hair. These are attached to easily-detachable "links" of cactus, which break off so they can take root elsewhere. What this amounts to is that if you so much as brush the things, you'll end up with a three-inch-across ball of spikes attached to your leg. Even once you manage to get the main body of the thing off, it can take hours to dig all the spines out, since they're very difficult to see. Forget the cacti you see in cartoons; these are the ones you have to watch out for. Fortunately, they are highly flammable
Equally unpleasant is the 'tiger pear' cactus, Opuntia aurantiaca, a South American species. They're invasive, grow fast, and have long, sharp barbed spines that can punch through truck tires, shoe leather, and anything less tough than those. The pods with the spines detach, so simply walking away from the plant doesn't do anything to relieve the painful stab wounds you just got. They're also surprisingly stealthy since they're not huge, cartoonish attention-getters like the saguaro or beavertail cactus, so it's possible to brush against one by accident and pay dearly for it. Worst of all, they're absurdly difficult to kill. One or two isolated plants can be destroyed by burying it or judicious amounts of gasoline and a match, but when it's a whole field of them, well, good luck. Spraying becomes a precision and accuracy problem due to the need to kill the extremely tough root of the plant, fire is no longer as effective for the same reason, and while insects are effective, they are alternatey unreliable or slow. The only solution is to constantly plow the invaded area, since few plants can exactly handle being constantly dug up and unable to reproduce.
Wasps in general, especially if one stings you (or you make the mistake of killing one) in the vicinity of a nest, in which case, hope you're in good cardiovascular shape. Beyond that, four species are particular scary:
Yellowjackets, in spite of being one of the smaller species, are more numerous, due to being A) smarter, and B) more aggressive. Also, they're carrion eaters, which means, if you're in the woods alone (a dumb move anyway, but that's beside the point), and they sting you to death, well, you can probably see where this is headed...
Bald-Faced Hornets. Not quite as smart, or numerous, but definitely as aggressive.
Pepsis Wasps, aka Tarantula Hawk, so called because they hunt Tarantulas. While not particularly aggressive, their sting is so painful, it pretty much shuts down your ability to do anything besides lie down and scream. You know Fallout: New Vegas' dreaded Cazador? This is what inspired it.
Asian Giant Hornets. Three inches long, with venom that can dissolve human flesh, if that wasn't bad enough, said venom also doubles as a pheremone that will attract every other giant wasp in the vinicty and they can spray it like a cobra! And they eat bees! Fortunately, they only live in Asia, but, apparently, they're everywhere there. Thankfully, they're not aggressive unless the nest is threatened.
Bullet ants. Found in South American rainforests, they're named for the fact that their sting feels like a gunshot wound. Some indigenous tribes have a lovely little ritual wherein a boy must wear mittens with bullet ants sown in stingers-inward and keep them on for ten minutes. And they have to do this 20 times over a few months or years to become warriors.
Another horrifying species of ant is the army ant. Enormous colonies of these guys travel along the ground, devouring almost anything unable or too stupid to get out of the way. And if you happen to be something that they don't want to eat, they'll still bite the everloving hell out of you and leave you for the scavengers and flies that tend to follow them around.
Africanized honey bees. Fortunately however, they only spawn in certain locations. For now.
Far worse are Jack Jumper ants, since one sting can send someone into shock. Then there's siafu ants, the one ant that actually predates people.
To elaborate: colder temperatures used to be effective in keeping "killer bees" from going too far north. Now, however, they're slowly gaining a tolerance to cooler climates, and thus are beginning to spread. (not sure if the same is true for the ants, though...)
Oak Processionary Caterpillars are the worst of all the insects, their bodies have 63,000 toxin filled hairs that make them crawling biohazards, a simple gust of wind can carry the toxins throughout the air, causing asthma, skin abrasions, anaphylactic shock, blindness and even death. They became such a problem that the Belgian government had to call in the military to get rid of them with flamethrowers.
Mosquitoes. We've all been bitten by them right? Well, through spreading diseases, mosquitoes kill more people a year than any other animal.
Alligators. They're large, extremely tough, and strong, and one bite from them usually ends up in agony, a missing limb, death, or all three, because of their infamous "death roll." During the mating season, they get even more defense and agressive towards anything. And because humans are encroaching on the (American) alligators' territory, they show up all over the place in southern Florida, especially Miami. It's not uncommon for an alligator to enter a person's backyard over there. Now you know why the reptile-control people are working overtime.
Alligators usually won't attack you unless you provoke them, crocodiles on the other hand are perfectly content to attack and kill people unprovoked. Saltwater and Nile crocodiles are the most deadly, the former species has the infamous "Gustave" a large crocodile roaming Africa that has killed over 300 people to date, and Nile crocs are infamous for attacking campsites and killing campers.
Since the alligators' eyes are low-set, they have trouble seeing up and will usually run away when they see legs of a giant (i.e. human). However, if you feed them even once, they may lose all fear of humans and assume every human will have a snack (or can become one). This is why in states like South Carolina, it's highly illegal to feed a wild alligator.
Suffice to say, there's a reason that the sole difference between alligators/crocodiles and their ancestors is size.
Some pathogenic microbes that are fairly ubiquitous, hard to detect, and lethal with a week.
Naegleria fowleri lives just about everywhere and infection is rare, but warm water can act as it's vector. If it enters through the olfactory mucosa or cribriform plate it can access the brain pan were it becomes pathogenic causing immense damage to the central nervous system with primary amebic meningoencephalitis. Translation: It swims up your nose and eats your brain, swiftly resulting in death.
The Ebola virus. No cure, can show its true colors via organ failure and internal bleeding in a mere few days, has a 50-90% chance of killing you, can't even be seen without a microscope, and is probably airborne according to a late 2012 study.
Drones. They are small, relatively inexpensive, can be entirely automated, can be deployed in places impossible for traditional means of warfare, and since they aren't humans, they will never be traumatized and— this is important— won't ever testify in the International Criminal Court. Every country worth its salt is researching this technology. You say war never changes? Well, it's changing right before your very eyes.
Any special forces unit would qualify for this status in the eyes of their enemies; examples are the US SEALs, Russian Spetsnaz, or British SAS.
The AC-130 a massive plane with enough firepower to level entire armies on the ground. In Afghanistan, and Iraq this is the one plane that terrorist fear the most besides drones, when they see it they start running to the nearest cave to get out of sight.
Snipers in general. Nigh-impossible to spot (they're either half a mile away, camouflaged, or both), and they can deliver a One-Hit Kill with nearly no warning.
During the initial stages of the Soviet invasion of Afghanistan, the Mi-24 "Hind" helicopters were this to the Mujahideen. While very heavy, they could still show up and rain rockets and machinegun fire on the lightly-armed Afghans. However, this pretty much stopped once they got their hands on some Stingers. Then the heaviness of the Hinds proved their undoing, as they were too cumbersome to evade the Stingers.
Torpedo Boats. Essentially, speedboats armed with torpedo launchers. With advances in steam engine technology in the late 19th century, these little guys, in sufficient numbers, presented a very real risk of overwhelming the larger battleships and cruisers, their torpedoes being capable of causing tremendous damage against the larger, slower ships. This resulted in the creation of an entire type of (now quite omnipresent) warship, the Torpedo Boat Destroyer, which over time had it's name shortened simply to Destroyer, whose traditional role was to protect the larger ships from torpedo boat and submarine attacks.
Just as often inverted through sheer tenacity (Russia and Finland in WWII, Vietnam, Bosnia, any conflict involving Afghanistan) since serving your country by defending your home and family is always a greater motivator than serving your country by going halfway across the continent or globe to kill people for ideological reasons. When insurgent armies can strike anywhere without fear of enemy fire and kill an entire squad before conventional army units can respond, they turn into this trope.
In most modern wars tanks fill the role when combatants lack anti-armor capabilities or tanks of their own.
Pirates are these when you're traveling in the ocean with no means to protect yourself