Irrational Childhood Fears (*cue spooky tun-tun-tun music*)
Clowns: Something about clowns really rubs me the wrong way. Even before the days of John Wayne Gacy, I was always a bit twitchy about clowns. Maybe it was because they always seemed to try too hard for laughs and this came across as insincere.
Marrionettes: Not all of them, but one in particular creeped me out. We used to have this old beat up marrionette that was supposed to be a cute musician type doll. Unfortunately he'd seen better days and by the time I came around his face resembled Freddy Krueger and the formerly charming smile looked more like a leer from some evil little thing.
Zombies: The first of my quasi-rational irrational fears. Zombies were scary because they worked as a large group and you couldn't always reliably stop them (a la Return of the Living Dead). This is probably why I got such a kick out of revisiting the old Romero movies and the new re-make of Dawn of the Dead. They don't qualify as an irrational adult fear because I'm pretty sure they don't exist, but as a concept they're still creepy. As one character observed in Day of the Dead, if the world was overrun with zombies, "where would you go?"
Werewolves: For a while, I was obsessed with the old MGM monsters from the 50s. This eventually grew into my current fascination with traditional "monsters" but when I was 10 or so, I went through the list of critters and realized that werewolves were the only ones that really scared me. Vampires were nifty since you remained you and could pick and choose who you killed. Frankenstein's monster and the mummy were silly because they could mostly only catch you if you tripped and hurt yourself (a brisk walk would keep you safe from them), etc... Werewolves, otoh, were always dangerous and having grown up with dogs and knowing how fast they were I knew there was no way I could legitimately outrun a werewolf and carrying silver weapons was unlikely to be practical in the long run.
Big Bugs/Spiders: No, we're not talking ginormous "attack of the 50-ft _____", I'm talking more like, small dog sized, maybe a little bigger. Big enough so their claws, pincers, etc... would be big enough to do real damage, but small enough that they could still skitter around quickly. Remember in the old horror movies creatures got much slower the bigger they got so a lumbering building sized insect or spider isn't quite as threatening as a swarm of fast moving insects/spiders that are 4-5x their normal size.
Yeah, I was simultaneously and overly rational little kid with a very over-active imagination.