>>51839926real but never confirmed by a ruby and sapphire beta release
"In the May 2007 issue of Nintendo Power about Pokémon Diamond and Pearl, Junichi Masuda stated, "There's also a sea-slug Pokémon that we weren't able to put in Ruby and Sapphire that we were able to put in this one...", which likely suggests that Shellos and/or Gastrodon (existing sea-slug Pokémon) were planned for Generation III instead of Generation IV.[1]
Supporting this theory is the fact that internal data in Pokémon Diamond and Pearl reveals a back sprite for both Shellos and Gastrodon with a slightly different design, but no front sprite. Shellos's sprite is pink, like its West Sea variation, and seems to combine the West Sea variation's head with the East Sea variation's body, while Gastrodon's sprite is brown and looks like its West Sea variation, but with more rock-like protrusions on its body. "
