jellyfish. they're not fish, they're inverbrates (most are cnidarians). jellyfish have been here way longer than dinosaurs. they don't have any hearts, brains or bones, but maybe that's what makes them so frightening...
a whale shark
deep sea animals scare me. have you heard of the vampire squid? vampyroteuthis infernalis...translation = 'vampire squid from hell.' it's so scary looking, i won't dare post a picture. this deep water cephalopod is so strange, taxonomically speaking, it hovers between octopus and squid in its own order (vampyromorpha)! it also has two blood red eyes that proportionally are the largest eyes of any animal in the world.
seahorses are funny. they are one of only a few species in the world where the male gets pregnant (don't let 'Junior' fool you). they also remain relatively monogamous.
don't try to eat a barracuda. their flesh can often contain ciguatera toxin, which can cause violent gastric pain and paralysis. barracuda can grow to be 6ft long and are fearless and ferocious predators. i once encountered a barracuda while snorkeling. i did the equivalent of slowly walking backwards...but while swimming.
gobies work well with shrimp. a perfect example of biological symbiosis. the shrimp maintains a burrow in the sand where both it and a goby live. what does the goby offer in return? it acts as a guard for the shrimp. you see, the shrimp has very poor eyesight, so if the pair are in danger, the goby flicks the shrimp with its tail and they both go hide inside their beautiful home. teamwork.
r.i.p. ophelia. and lola, and mr. orange, and jamie...