Mother fucking Brain

Andross. is a fucking dick! guy freaked me out, specially when he was a brain in the hard mode.

(Andross is the guy from starfox, right?)

Andross was the big flying monkeyhead with a creepy mask his back ... head. 

Atleast it was s SF: adventures

Dead Hand, battling this boss as a kid gave me nightmares.

Death Sword, broken neck, big black sword with a red epigraph, how can you go wrong?

Koloktos, not that scary but his laugh sure gave me the creeps.

Lol, when I saw the thread title, I actually thought it was about Nintendo's IRL bosses.

But if it's about in-game bosses, then the first boss in OoT was pretty scary when some of my friends and I were playing it around the time it was new... The spider thing, I think it's Gohma or something... It was just so epic, dropping eggs after a while, hatching into enemies. After that boss we were prepared for anything though.

The lack of images in this thread is the scariest thing of all.

Damn, you beat me to it =/

I checked quickly to see if anyone else had put him in the thread yet - I m sure i am not thel only one to think of him for this - Maybe he really IS the scariest boss-  I think so(:

Don't worry, I am your saviour.

Bowser from Yoshi's Island lol.