Honestly, I haven't played much horror themed games, so my list may be...It's not very scary. In no particular order, here they are: Marx Soul The messed up boss from Kirby Super Star Ultra, it make look all right in the picture, but it's a pretty disturbing thing in action. just look at the eyes... Laughing Octopus Each of the bosses in MGS4 had their own emotion theme. Laughter was the creepiest :| Dark Beast Ganon He disappears...you know he's there...somewhere....behind you! Giratina It's all about the eyes...he controls the Distortion World. GLaDOS Throughout Portal, she keeps getting inside your head...it messes you up... Tabuu The manipulative final boss from SSBB. It is the master of the dreaded butterfly attack. Dr. Fetus A fetus in a jar... Twinrova You first fight two twin sisters, Kotake and Koume, who each have the powers of Ice and Fire. In the description for both of these sisters, it says that they are the surrogate mother of Ganondorf...they can actually fuse to form one boss called Twinrova. This boss is ugly and sick. Mimi ....why are you looking at me like that? This girl is creepy... seriously. She's just sweet and innocent one moment...then she stops...her neck cracks, and cracks...the music box type music plays...and her head twists. Suddenly, you're facing a giant, eye-less spider. Mimimimimimimimi!
EDIT: I had 9 bosses... so for my final tenth boss.... - - - - - - Snivy is boss! Fear me! Mwahahaha!