Any enemy ever that relies on unpredictable movement patterns and unpredictable attack patterns.
For platformers, this goes to:
-SMB1/LL Hammer Bros. No amount of practice will prepare you for the hell that's these enemies.
-Lakitu. Lakitu has got to be the biggest troll in the entire Mario universe. No matter what universe he's part of, he'll screw with your head. He'll snipe you randomly in the classic platforming games, and he's deceptively accurate at it too. In SM64 however, especially in romhacks, he'll show up out of nowhere and snipe your jump by just existing. He doesn't even need to throw a spiny at you to do this too.
-Fishin' Boo. This is basically Lakitu if he was dead. Maybe he should've been better off being left alive. RAGEEEEEEFKLGHEDTGLJEKHFTDDKHGEIEHTIETHEITFUKKKK.
For RPGs, this goes to:
-Any regular, encounterable enemy who has access to instant death or petrification, and appears in groups. I'M LOOKING AT YOU FF1 COCKATRICES!
-Any enemy in a game revolving entirely around capturing or gathering that has a sacrificial move. I'M LOOKING AT YOU, POKEMON, WITH YOUR GEODUDES AND ELECTRODES!
-Extremely evasive enemies for no rhyme or reason. See Also: EVERY ENEMY IN FF6 MT. ZOZO
-Any enemy that relies on RNG in general, either through dodge rates or crits. I'm looking at you, EVERY F'ING SWORDMASTER AND BERSERKER TO EVER EXIST IN EVERY FIRE EMBLEM EVER. ESPECIALLY THOSE WITH WO DAO OR KILLER WEAPONS.
-Boss-In-Mob's-Clothing. If you're going to make a regularly-encounterable enemy that has the potential to party-wipe me, at least make it fairly obvious. I don't mind if it's an extremely rare encounter, but if you're going to field this enemy under a normal encounter rate, then whoever designed this is a goddamn troll. See Also: Greater Mantis from FF6.
TL;DR I'm not a friend of RNG, and RNG is not a friend of mine.
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STELLA!