Commander Keen/Nightmare Fuel

• The vacant grin and huge eyes of the Dopefish terrified some younger gamers. *cough* The bouncing mushrooms with insane faces from Secret of the Oracle were also a bit... disconcerting.
• Boobus Tuber, a giant potato with a lot of eyes that is several times bigger than Keen.
• Imagine being out of ammo in the first game, and you're running away from a Garg. You come to a pit and so you jump over it, fully expecting the Garg to fall into the pit. Imagine to your horror that Gargs can jump over pits and continue chasing you...