Baby Jellies by Peter Willows/BNPS, telegraph.co.uk: Hundreds of moon jellyfish babies have been born at the Weymouth Sealife centre in Dorset. Aquarists say they have never seen so many jelly babies of all shapes, sizes and colours from many different species at one time - but even though they may look cute many of them are highly poisonous as well. #Moon_Jellies
Glaucus atlanticus "Blue Dragon" nudibranch. It spends its life floating on the surface of the ocean. They feed on the poisonous Man of War Jellyfish, collecting the animal's toxins and storing them in little sacs on the end of their feather-like "fingers." The Blue Dragon can produce a much more powerful and deadly sting than the Man of War due to its ability to store the poison.
Physalia physalis, a.k.a Portuguese Man'O War. It's not a "simple" jellyfish, but the symbiosis of more specialized foms of life. In the best case, its urticant, toxic touch burns like a flame [TLDR version: pretty, but don't touch!]
Anyone unfamiliar with the biology of the venomous Portuguese man-of-war would likely mistake it for a jellyfish. Not only is it not a jellyfish, it's not even an "it," but a "they." The Portuguese man-of-war is a siphonophore, an animal made up of a colony of organisms working together.