A pokemon, maybe even a pseudo legendary, based on the glaucus atlanticus, or blue sea dragon. These animals already look like straight-up pokemon. They feed on the man-o-war siphonophore and use the dangerous stinging cells of the man-o-war for their own defense. I may even design the man-o-war pokemon myself but I leave this one up to you
Pokemon itself: Possibly water dragon, poison dragon, or water poison type. This pokemon starts off pretty small, letting the sea carry it around. As it finds and consumes its prey, however, it incorporates the prey's body mass into its own "wings." The more prolific the hunter, the larger the pokemon. If its wings grow large enough, this pokemon can glide short distances, occasionally carrying its pre-evolutions on its back to show them the skies they could soon inhabit.
Name ideas
First-form: Slue, Bluup (pronounced "bloop"), Atlip
Second-form: Stinbit, Atlent
Final form: Siidrake (Pronounced "sea-drake"), Atlabranch (pronounced "brank" like nudibranch)