ReDeads. They terrified me so much that I almost wasn't able to beat the Shadow Temple or the Bottom of the Well. And I used the Warp Songs to avoid going through the ReDead infested Market.
I even sent an e-mail to HTLOZ asking them how to get over my fear of ReDeads. Falco just responded "I need a vacation..." Which, needless to say, was totally unhelpful. There are now some videos on YouTube that give helpful procedures for conquering ReDead fear now, though... back then, I ended up driving myself insane and imagining ReDeads were around every corner in real life. Eventually, I got upset with the game. I beat the game in order to deal away the evil, and then sealed my OoT cartridge in a Wooden Box that I labeled "Pandora's Box" for several years to keep the ReDeads from coming out.
My mother says that I seem to get very upset when I lose at video games ever since then... almost as though I take the game personally, or see it as life or death on some unconscious level. It could be a side effect of trying to cope mentally and emotionally with the concept of ReDeads when I was 11 or 12.