How did I just realize this thread existed?
Well, since I didn't notice it before, I'll post now. And I'll throw in a 3d platforming boss, and even an MMORPG boss because I haven't seen that many of either.
And from some games I don't think too many people have played.
Pac-Man World 2 - Cylde (It's actually Blinky, they messed that up somehow)
This boss is a mess. Unlike every other boss in the game up to this point, you cannot roll into him at all to damage him, which made all the other bosses in the game pushovers, so you can't do that, to start. You're on a secluded island surrounded by lava, the only other way to another platform is to rev-roll to the one of them (all other bosses had platforms going over to their other sides, this one doesn't, miss and you die.) He attacks quite fast and every attack of his leaves flames on the ground that linger for quite a bit and can still damage you. So you have to dodge every attack while also not accidentally running into the residual damage on the ground after he attacks. The only way you can hit him is to wait for him to stop attacking and give you an opportunity to attack by bouncing on top of him, which wouldn't be too bad, but his hitbox shrinks every time you hit him! So by the end you'll likely be spamming just hoping for him to die! The only saving grace is this game is very generous with it's boss checkpoints, if it wasn't... ew.
There isn't very good quality videos for just this boss fight, but I'll try to find one anyways. I only found out through this video that he's somewhat cheeseable if you don't move too much, because he isn't super accurate if you don't move... still a pain in the ass though...
And since I really love Runescape despite all it's flaws, it's still my favorite MMORPG and the only one I ever liked (At least old-school), so I'll post a boss I don't like from it!
Runescape - Treus Dayth
Boss of the Haunted Mine quest, a relatively relaxed quest that's basically just a chill puzzle up to this point. The boss is only level 95, which is honestly really low, especially for experienced level quests. Which would lead a player to think this fight wouldn't be so bad.
This fight is a pain in the ass. You fight in the middle of a room filled with boxes, cranes and minecarts. The boss barely ever directly attacks you, but he just constantly runs away, attacking you with constantly teleporting pickaxes barraging you and he possesses the cranes and minecarts to hit you whenever you pass them. He doesn't have too much HP, but you will barely get many chances to attack him because he's constantly running away and stops for barely any time to actually hit him more than once or twice. You have to chase after him and avoid the cranes and minecarts as much as you can, especially the minecarts.
Everything hits quite hard, harder than you would expect, cranes can do a number on you, and if you get hit by a minecart and don't eat enough, you are dead, because the minecarts combo you super hard and do so much damage that if you don't have enough food to eat through that combo damage you're dead, or even if you just aren't expecting it.
And if you don't have stamina potions it's very easy to run out of run energy and basically forfeit your attempt.
Started playing Runescape again, I found out this boss still is annoying even if you know it's coming and what to do. Very cool.
Found a decent example video for this boss.