I used my Vaporeon to use Surf and then it turned into a Dewgong-Like thing and it occurred to me, why does it look like a Dewgong when it's a Vaporeon? The same goes to all my Pokemon that use Surf.
They just made it like that. All generations have a set sprite for the Surfer. The Dewgong lookalike just happens to be Generation I's Surf Sprite.
Gen I-Gen V: