Gears of War - The Kryll. Step out of the light, one beat goes by, then "WHY AM I DEAD??." "Goddamned space bats."
In Gears 2, there are tickers. They are like the lambent wretches, except they are smaller, faster, and blow themselves up behind your cover as their standard attack. The same rules regard damage apply. And to have these going off during a firefight with Demonic Spiders...
Gears 3 also adds polyps and lambent humans to the mix, both of which have a nasty habit of appearing out of nowhere during intense fights.
Warhammer 40,000: Space Marine - tired of fighting the Goddamn Orks? Us too, let's kill some Chaos heretics. Too bad the Chaos Space Marines are surrounded by Renegade Guardsmen and Bloodletter Demons. The former will abuse grenades and Beam Spam to the nth degree, while the latter will charge at you in packs and stunlock you with their huge flaming swords. And Bloodletters can teleport out of the path of your bullets. And some Renegade Guardsmen turn into Bloodletters when they die. Finally getting the chance to Execute the bastards is one of the best feelings in the game.
Sappers in Tomb Raider (2013). Their molotov cocktails force you to keep moving from cover to cover, which besides exposing you to enemy fire, makes it very challenging to draw a bead on a target since you're constantly having to reposition to not get blasted. You have to be able to aim and shoot very quickly, (god help you if all you have is the bow and its very low rate of fire) and it certainly doesn't help that they have arms like Major League pitchers and almost perfect aim. Sappers will quickly make gunfights insanely frustrating, especially if there's more than one of them. They get worse when you start running into the ones that carry dynamite, which graduate into outright Demonic Spiders.
The Grineer Rollers are tiny metal balls that roll around stages at high speeds, turn on a dime and stun you with each attack. There's no way to take them out silently, so encountering one is likely to ruin any attemp at stealth. One player observed that all the Grineer would have to do to conquer a planet without sending in troops would be to drop a handful of rollers from orbit and the entire planet's inhabitants would very quickly pack up and leave.
The Grineer Seekers are green-colored soldiers that constantly throw mini versions of Rollers called Latchers. If a Latcher rolls into you, it sticks to you and detonates in a few seconds, unless you dodge to dislodge it.
Scorpions are Grineer melee fighters, who by themself are not more dangerous than the standard Grineer Butchers, but they have a 100% hit chance with their harpoon special ability which throws a Tenno to the ground and pulls them towards the Scorpion.
The Corpus Osprey have no attacks, but project a shield around all their allies and fly around in random patterns making it hard to kill them. They are so annoying that players might charge into formations of enemies just to take them out for the rest of the team. Like Rollers, they cannot be back-stabbed, guaranteeing they will alert guards to your presence, whethere by noticing you or by being destroyed.
Corpus Shockwave Moa can knock Tenno down. One by itself is still okay, but when there are multiple it gets worse. Even worse if you're running solo and they stunlock you to death.
The Corpus Ratel, spawned by Sniper Crewmen, are tiny rolling robots that emit an annoying electric beam to a target within their range. They chase their targets relentlessly at high speeds, and will spawn in groups of four if you don't stop the spawner.
The open-world section of Deimos introduces a new kind of flying pest, the Deimos Tendril Drone. Tendril drones aren't particularly powerful or particularly tough. They do little damage and can be easily snuffed out by a few pistol shots (or a shotgun if you have one handy). However they're not pests because they can kill you, they're pests because they're flying Face Huggers that inflict pure Interface Screw. They turn off your Warframe abilities, cause the entire UI to flicker and warp, unavoidably stagger you (such that the Sure Footed mods and other abilities that prevent stagger are flat out ignored), and rip you out of your Archwing or K-Drive mode if you are using those gear items. Oh, and the little bastards home in on you too. The one saving grace is that removing them is as easy as dodge rolling, and they have the common decency to announce themselves with a hellish screech before latching onto your head.
Decker Specialists in Saints Row: The Third rarely stay still for more than a few seconds, and when they move, they move fast. It's hard to even hit them, and they can show up in groups or with other Deckers, and it can get overwhelming fast.
The book bats in Ghostbusters: The Video Game spawn in massive hordes. You will not get a moment to breathe with them around, which can quickly turn them into Demonic Spiders. They can be destroyed... but good luck hitting a small, fast-moving target with your decidedly unwieldy proton stream!
Smallfry are the weakest enemies in the mode, but they're deceptively fast and due to being so small, they can can be hard to see. This means that they have a tendency to sneak up on you to do additional damage while you're busy dealing with other foes.
The Drizzler attacks by launching a cloud that rains ink on the stage. This attack is so weak that it probably won't kill you unless you're already on your last legs, but it covers a wide area and can severely limit your mobility, which is a big issue if you're trying to deal with other bosses. Making it even more annoying, the Drizzler is invincible most of the time, only leaving its shield while moving or immediately after firing its cloud bomb.
Splatoon 3 adds the Big Shot, which uses a cannon to launch shots towards the egg basket that create shockwaves that can damage you if you don't jump over them. The Big Shot itself won't attack you directly, but it can launch its attacks from any shoreline on the stage, and unless you see the shot land, there's no way to tell where they are. Once you locate them, it turns out they can take an enormous amount of punishment and they constantly retreat into the water (where you can't hurt them) to retrieve more ammo, and that's not even taking into account that other enemies will constantly spawn from the same area. The shockwaves they create don't do a lot of damage, but jumping over them makes it harder to maneuver, and because they aim for the Egg Basket, it's guaranteed to make an already high-traffic area even more dangerous to navigate.
