Poisonous bats! And vampires.

Fleamen AKA controller snappers supremus.

The Quick and the Dead:-)

https://www.youtube.com/watch?v=q6cyDsuNx_U

Medusa Heads in Castlevania games without a shadow of a doubt. Trying out hard mode in Castlevania: Order of Ecclesia was the ultimate exercise in frustration because of those

Those vampire warriors in the last few levels of Severance. Those things can both teleport, and drain your health when they hit you. Means that even if you get them down to a few hit points, they can just nip behind you, hit you a few times, and you're back to square one.

Oh, those vampires... annoying as hell!
When playing as the barbarian, though, their teleport ability is actually a weakness: that character has a 180° move that allows to quickly turn and hit the enemy at his back. Since the vampire lower its guard when reapperas behind you, killing it becomes very easy!

I'll leave this... ;)

About the topic, I'll saw which ones annoy me the most.

- "Vendor trash" Pokémon which can't be one-shotted because of an ability such as Sturdy (Geodude, Roggenrola, etc.) or a move such as Protect (Pelipper). Or Pokémon which inflict status ailments just by making contact with them (like those with Static or Effect Spore). Or those which encounter rate is ridiculously high (Zubat, Tentacool, I'm looking at you).

- Cazadors in Fallout New Vegas. Really. I remember a run where I loathed them the most, as I rolled a character who used light armor and hand to hand combat. Poison and broken armor were something common.

- Basilisks in Dark Souls. I remember the first time I encountered them. I went head on to them and they exhaled their nasty breath. And my curse resistance was low, so I was cursed. Argh. And curse is a nasty ailment in Dark Souls.

- Marlboro in Final Fantasy X. When I first encountered them they were tough and their breath was awful. My party got confused and slaughtered each other. Nasty.

- Those spirits in Diablo II which drained all your mana. Elite mobs which exploded in a frost nova too.

- Barnacles in Half-Live. First time I encountered them in the Uplink demo I thought it was a climbing rope. I died and I still can't get over it. :(

Boss enemies that break the rules of the game, like the final in Two Worlds 2. Have 100% fire resistance? The fireball will still kill you in 2-3 hits. You have ranged attack powers that could level a city? You still have to use those fucking ballistas that make you a sitting duck while you draw the string...

The Spawns in Quake, small and fast and irregular movement pattern. And splash damage when dying O_o.

Big spiders, especially when they make clicky noises.

Mountain lions in Red Dead Redemption and GTA V. If you don't constantly stare at radar, they come out of nowhere, swipe you, and you're luck to get a shot off before they kill you.

I never really liked boss battles.

Yes, you'd have to keep running for the rest of your life, eventually, you would succumb and become one of the living dead, but at least you gave it the good old "college try".

Ornstein & Smough in the first Dark Souls game, I was stuck at them for around 3 months till I managed to summon 2 human players. There was also the 2 black knights with the dragon bows on the battlements you had to run up - now that was bloody cheap.

Not forgetting the fight with the Capra Demon in the tiny area, 2 handed attack as soon as you enter the fog gate, once again really cheap if you aren't aware it is coming and not forgetting the 2 bloody dogs.

Welcome to Thunderdome!

Made my day :'D