One of the earliest mythological tales I encountered was the surprisingly sexy tale of the fisherman, Glaucus. This was the first real transformation fiction I had read that made me envious. I'd never really thought vampires and werewolves creatures I envied, one was a blood sucking fiend and the other was a hapless soul who lost any true sense of self every time there was a full moon. There wasn't much there I admired. Glaucus' change, on the other fin, was something to relish.
Glaucus was out fishing at his favorite spot on an off-shore island when he noticed that some of his catches were somehow managing to revive themselves, wriggle from where they lie upon the grass, and leap back into the briney. Curious at why this was happening he chewed a bit of the herb himself and quickly developed a tremendous desire to go with the fishes. And so he did. And, once there submerged in the liquid blue, Glaucus experienced a fabulous transformation wherewith he became a green-skinned, fish-tailed, kelp-locked muscle boy and scaly sea deity. A sort of merman in excelsis.
At first he was shocked by the development, but Glaucus soon became enamored of his new form and took to the sea he had always loved as his great green and blue playground. He reveled in his new found strength and abilities.
Then, as these tales need to impart lessons so never have happy endings, he chanced upon a sweet young thing named Scylla and, though now a sea deity but still a male fool, he quickly fell head over tail in love with her. And, guess what, the love was not reciprocal. Glaucus considered himself a good catch--why the Catch of The Day for any day!--but Scylla thought him monstrous and that was her incredibly unfortunate downfall. Picky bitch. (You can read a glib telling of the tale here.)
Anyway, I don't want to go into the gory details. Unfortunately, Glaucus is not a highly popular artistic subject and good representations are few and far between to say the least.
But here is a nice one:
And another more current interpretation by Carlos Paradinha. Who just so happens to have a bevy of beautiful manimals featured on his website.