White-tailed Deer are a perfect real life example of Goddamned Bats. At night, they usually run right into your car when you drive and they're hard to see, depending on how little light there is at night. To make their status known, they have severely injured or killed drivers because of how unintelligent they are.
The deadliest animal in the USA is the deer, simply due to causing so many automotive collisions. A supposedly harmless creature that causes a disproportionate amount of trouble is pretty much the definition of this trope.
And then there's their Demonic Spider counterpart: the moose. They have all the same traits of the white-tailed deer with regards to cars and the main body of the moose is above normal headlight height with only the spindly legs in view. As these are hard to see, people will often run into them, breaking the moose's legs and sending the rest of the much bigger animal right into the windshield. Ouch.
During the Yukon gold rush, this got turned Up to Eleven. One of the biggest dangers a train and its compliment of passengers could run into is if there was a moose on the tracks. There were a few notable train accidents from the era that were caused when striking a moose caused the train to derail.
Mosquitoes. Fucking mosquitoes. Not only do they suck our blood and itch, but they're small, hard to detect, and the very noise they make is obnoxious. BZZZIIIIIIIIIII-
Or flies, or, for pet owners or those who live near animals in most areas, fleas and ticks.
On the topic of insects, one mustn't forget ants.
And cockroaches; for every one you kill. three more crawl out of nowhere.
Mosquitos are Demonic Spiders in some parts of the world since they also carry malaria.
Deer flies. Their purpose in life is finding a deer or another bovine, sucking their blood and then laying their eggs on them. Unfortunately, they tend to land in human hair just the same. Fortunately, they won't do the "suck blood, lay eggs" routine then, but what they do do is walking around one's scalp and itching like crazy. Picking them out can be quite a hassle, especially for those with long and/or thick hair.
There is anotherDeer fly that basically behaves like a mini horsefly, and several can buzz around your head at once. Worst of all, no conventional insect repellant will work against them, with a special sticky patch you put on your hat being the only effective solution.
Horseflies and their relatives (also confusingly called deer flies) have a bite that's quite annoyingly painful, and if you try and wave them off, they land right back where they started and bite you again.
Stink Bugs are a major headache in the United States and parts of Asia.
Rabbits for those with gardens, because they keep eating all your damned plants.
Rabbits are an invasive species in Australia with a population that spun horribly out of control after being introduced. They're still so numerous that there are essentially no restrictions on hunting/killing them, and the government is struggling to eliminate the infestation.
Canadian Geese. They are everywhere. One of the most common birds, especially in places such as Ohio.
In one infamous incidence in 2009, a flock of Canadian geese somehow got into an airplane's engine while said plane was flying over Manhattan. If the pilot didn't have military training and manage to safely land the plane in the Hudson River (no deaths!), we could have had ourselves another needless airliner tragedy.
For those in cities: pigeons. For those near a shore/on an island: seagulls. For those in Minnesota, and New York, both.
In selected cities, however, pigeons are viewed much more favorably than an uglier, nastier, more invasive type of bird: grackles. They're huge, bullying, greedy, obnoxious, and nasty black birds. Pigeons are downright pet-worthy compared to grackles.
For Australians that live on the Queensland coast, bush turkeys. They're everywhere. To make matters worse, they incubate eggs by making a compost heap and laying in the warm, rotting vegetation. If you have anything remotely resembling a garden, they will earthmove all your mulch and plants to their nest. They can destroy an entire garden overnight, moving all the mulch and seedlings across a road and into the nature strip.
On the subject of horrible things that live in Queensland, Cane Toads, an invasive species introduced from South America to control bugs which proved to be far more of a pest problem than the bugs ever were. They're ridiculously hard to kill and very, very poisonous, which has done incredible amounts of damage at all levels of the ecosystem. Generally speaking, Australians do not like them. At all. Finding ways to kill them horribly is practically a national sport.
Physalia physalis, also known as the Portugese Man 'o War or the blue bottle. Unless one has an allergic reaction the worst this thing's poison can do is cause severe itching for a while, but when carpets of them drift onto a beach the reaction is best described as "Goddamn Blue Bottles!".
Made worse by the fact that its tentacles can still sting even if they aren't attached to anything, so you can randomly be stung by what looks to be nothing more than a piece of blue string. That's right. They can turn into invisible Goddamn Bats.
Horse archers, especially of the steppe variety, initially seemed like goddamned bats. They would gallop in and out, harass an opponent and generally made themselves as annoying as possible. Once an army broke formation they would surround it and suddenly turn into Demonic Spiders.
Asian/silver carp are becoming these in the Great Lakes region. They aren't actively predatory (in fact, they don't really need to move in order to feed; they essentially filter-feed), but they're crowding out the native population to the point where there can be up to and over fifty carp for one of a native species in a particular area. It gets worse.... the damn things jump like fleas when panicked, and a forty-or-more-pound fish to the kisser will ruin anyone's fishing trip. Think Magikarp's flailing barely hurts? An Asian carp's flailing can break bones. And to cap it off, nobody's interested in them for food because they spoil much more rapidly than other species, making people think they're inedible. (They aren't.)
Head lice, not as bad as ticks but they can latch on anything, are so damn hard to remove. They can latch on your clothes, and hold on to them in a washing machine and survive. The most practical way of removing them is to pick them off one at a time.
Squirrels. Like deer, they have a tendency to run head-first into the streets. Given that they are much smaller and faster, you may not be able to see them in time when you jam on your brakes.
Sadly, dogs and cats in some places. There's a reason Bob Barker told people to have their pets spayed and neutered every single day for decades. Here's the story (for the sake of simplicity the pet is a female dog): A family gets a new puppy, but doesn't get them fixed because they find it cruel. The puppy eventually matures and goes into heat. This will attract male dogs who do what nature intended. She then is pregnant with a litter of puppies. The family, who can't afford or don't want a full litter around, will send them to an animal shelter if they're responsible or simply release them into the wild if they're not. The litter eventually matures and each has their own litter. Now there are large groups of feral dogs that aren't adjusted to people (making them impossible to catch or tame), make endless noise all night, and will often tear into your trash for food and attack small pets. Animal shelters are log jammed, and it's telling that even the most hardcore animal rights groups still endorse spaying and neutering, it's that out of control in some places.
This is why scarecrows exist. Flocks of crows can come down and decimate whole sections of a cornfield. They can come in large numbers, are tenacious and worst of all are some of the smartest birds in the world.
Pinworms. They live in your butt and eat... the stuff that is in your butt, and at night they come out and lay eggs on your butt. This makes your butt itchy (they do it on purpose), so in your sleep you scratch it, getting eggs under your nails, and eventually put your hand to your mouth, EATING THE EGGS. The eggs hatch in the warmth of your body, and the whole thing starts over again. Even worse, if you're female, they can... "get lost" on their way back into your butt. If you want to get rid of them, you have to keep taking the worm-killing medicine every generation until you're damn sure they're gone, and make damn sure you've got clean underwear because they can hide out there, too.
Bed bugs. Yes they're real and not something made up from the bed time rhyme. They're tiny, hard to detect, and hitch a ride on everything you own. Detection is based on finding signs they were around (poop and exoskeletons they shed), and the fact that your body is now riddled with mosquito level itchy bites. And once you find them, it's too late. They also breed insanely fast, can survive harsh conditions and without food (up to a year), and extermination requires non-traditional methods because they disperse at the first sign you're poisoning them. Oh, and if you don't basically sterilize everything in the house including clothes, they'll come back.
We're about to get biblical here: Locusts. They come in swarms of thousands, sometimes millions (one swarm was estimated to have billions) and will eat any plant they come across. One day, your farm has beautiful green fields of vegetable crop. The next, a cloud of buzzing insects come by and the day after it looks like somebody's nuked the place. Fortunately, you can eat them, and the Islamic and Jewish faiths that normally prohibit the munching of bugs will let a locust lunch slide.
Gnats swarm in the thousands, maybe even ten thousands, and are everywhere during warmer months. Ever run into a field and see a huge black cloud rise from the grass at dusk? Gnats. Those black dot-like bugs you see flying around your house, especially around old garbage? Also gnats. They lay eggs in any soil and their larva eat the roots of plants. In grass this is fine and harmless, but if they get into vegetables, or important plants, it can cause them to wither, become sickly, or even die!
Termites. Every bit as numerous as ants, they are this to anything wood-related. From buildings, to timber, to paper, there isn't anything too thick for these critters to chew on. And some species are able to chew through metal, although fortunately, only a select few. So many millions of property money have gone down the drain thanks to these annoying little insects. Pesticides have been used to deter these persistent critters and yet, while it's seemed to have driven them back, they still keep on coming. Fire is probably the best way to fend them off but it's not a good idea when you're in an area full of grass, forests, or suburban abodes. You don't want to get gung-ho on your surroundings, do you?
Bats themselves for people exploring caves, or those who end up contracting rabies.
Spammers. Whether by email, social networking, or some other form of electronic communication, their purpose is to waste resources and your time, as well as junk your machine with viruses if you aren't paying attention to email attachments. While any email provider worth their salt has anti-spam measures, it can happen once in a long while that a spam message slips under the radar.
Spyware, the cousin to spam. Secretly installs itself on your system, and its effects range anywhere from bogging down your system to harvesting your data without your consent. Malware is a subset of spyware and is a different beast altogether.
Those ads that constantly appear for some sort of game heavily featuring some fantasy-esque skimpily-clad woman (the most common ones these days are for Wartune). They never go away, they're absolutely everywhere (heck, there's probably one of those on the side/top of the page right now), and they're a distracting eyesore.
Pop-up ads. They show up out of nowhere and disrupt your browsing experience. Some of these are "pop-under" ads; the rationale is that they show up without being disruptive, but browse long enough without checking these days and their resource consumption stacks up.
Hover-over text link ads. Even if you never hover over them, they are still quite the eyesore.