10. Ghost Fish - Ninja Gaiden
Ninja Gaiden is already a punishing series, so there's no real need for the ghost fish, which have haunted players across a bunch of the games, to exist.
These piranha-like scamps are invisible right up until they point where they're not, which is when they're just about to swarm you with a series of unblockable attacks. That's right, this isn't a case of panicking when they appear and going on the defensive, as they can unfathomably break through your guard and absolutely rinse you of health.
Until you really master Ninja Gaiden's gameplay and make it a point to nullify these bastards as much as possible, even experienced players will be caught off guard at some point, taking a few undeserved hits every time they show up.
Invisible enemies are bad enough, but ones that hit you no matter what? I'm calling the police.