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.
Sappers in Tomb Raider. 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 new Grineer enemy, the Rollers/Grinders. Unlike the Grineer, which are gigantic, hulking clones, these guys are tiny metal balls that roll around stages at high speeds, turn on a dime, and stun you with each attack. If they come in groups, especially near a Grineer heavy unit, kiss your life goodbye if you don't have an AoE attack primed. 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. Even worse if you're on a team, as they will shock and disable you until your allies manage to destroy the annoyingly durable and hard to hit thing.
A new addition to the Grineers Goddamn Bats are the Scorpions. Female grineer melee fighters, who by themself are not more dangerous than the standard melee Grineer. What puts them into this category is that they have a 100% hit chance with their harpoon special ability which throws a Tenno to the ground and pulls them towards the Scorpion. Since they can even hit players behind walls, it often means they are dragged into the fire of other Grineer.
The Corpus Osprey also count. They have no attack, 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.
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.
Decker Specialists in Saints Row: The Third. They 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. They 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!