As you can see, the Gastrodon on the right has a different head than the one on the left. However, Gastrodon-West only has one foreward sprite:
Explain!
Back sprites of Shellos and Gastrodon, presumably from one of the earliest stages of development, were found. These are possibly leftovers from when Shellos and Gastrodon were planned to be in Generation III. -Bulbapedia
So Basically they were gen 3 designs of it and GF never took them out when they remade Gastrodon/Shellos
http://bulbapedia.bulbagarden.net/wiki/Pokémon_Diamond_and_Pearl_beta Scroll down to sprites
