You're playing your way through a game, having a good time, when THAT enemy type appears. The one that gives you more trouble than a great number of bosses. And suddenly your day just got much less fun. You throw your head back and groan, burdened with the knowledge that somebody got paid to design this thing.
Yes, sometimes game developers design a baddie for the sole purpose of enraging their players. Whether it's through a high defence rating, strong offence, cheap attacks, or weird coding quirks, these are the enemy types that make you want to hurl your controller (or keyboard) across the room in a fit of rage.
First, a quick note. The "overpowered" facet of this refers to an enemy in comparison to other types within that specific game. So games like Dark Souls, where pretty much everything is hard to kill, won't be featured here.
The following entries aren't any special, rare types found in RPGs. Nor could they be considered a boss or sub-boss. Rather, these are the types of enemies that you're expected to take out regularly.