
The Giant Robot Wall Thing (Contra)

This is probably the only boss you will get to see without the Konami code.
Contra is notorious for making otherwise sensible little dudes swing their controller like a helicopter blade, smashing it to bits. Surviving hordes of enemy soldiers that can kill you with one hit can really raise your blood pressure to alarming heights. Luckily the most common boss in Contra was a wall. A wall that just sort of stood there, shooting just out of your range, giving you a moment to cool your thumbs. The best part about it: The red dude flying ass over teakettle off the roof when you bust a cap in his ass.
Koopalings (Super Mario Bros. 3)

This is why the Koopalings were never seen again after Super Mario World.
What could be better than Bowser as a boss in a Mario game? How about Bowser’s seven illegitimate children? Larry, Roy, Lemmy, Ludwig, Iggy, Wendy and Morton Koopa Jr. made their first appearance in Super Mario Bros. 3, offering no explanation as to how they came to exist in the first place. Perhaps the 7 mushroom retainers in the first Super Mario Bros. were actually midwives? That would explain why Bowser is so intent on kidnapping Princess Peach… she probably has some real child-bearin’ hips underneath that camisole.