These are artwork designs for Marlin the Merman and his human form. The one on the far left is his debut design. The one next to the first is his redesign for Season 4 before he became human through one of Professor Poindexter's inventions that alter DNA. And he'll be given the ability to switch back and forth by his father, Poseidon, so that he could travel between the surface and Atlantis whenever he wanted, as long as he comes into contact with seawater.