The Portuguese Man O’ War (Physalia physalis), also known as the Bluebottle, is a jellyfish-like marine invertebrate of the family Physaliidae. Its venomous tentacles can deliver a powerful sting.
Despite its appearance, the Man o’ War is not a jellyfish but a siphonophore, which differs from jellyfish in that it is not actually a single creature, but a colonial organism.
I saw these in a documentary about ocean life about a year or so ago…I still think they’re really neat.