I think it should start out poison, and goodra should be poison dragon.
Goomy is based off of gastropods like Slugs. However, it's design and later evolution into Goodra show that it is based off of the Blue Dragon Sea Slug. The name explains the typing, but if anything, you should be asking why Goomy isn't a Water Type since these slugs love in the Ocean and Goomy's Pokédex description mentioning it needing to stay hydrated.
Source: https://m.bulbapedia.bulbagarden.net/wiki/Goomy_(Pokémon)
