Clock spider was never really scary to me. It's been around the internets for about a decade. though.
Giant centipedes, giant jellyfish, and certain cephalopods tend to creep me the hell out. The first because they are fast, relatively large, super poisonous, needlessly aggressive, and invincible. One ran out from a plant at my friend vacationing in Hawaii. His brother picks up a shovel and smashes the damn thing with the spade. It barely phases it and it flails around for a second before running off. The second are stupidly big and deadly for something that isn't even bilaterally symmetric. The last because they have some alien lovecraftian intelligence and can communicate with chromatophores. The smaller they get the more poisonous they are, and the bigger they get the exponentially stronger they get.