Goodra, due to its Dragon type, elongated body shape, and retractable tentacles used for battling is probably based on the Lou Carcolh, a mythical serpent/mollusk creature from French folklore.
Like its pre-evolutions, it could also be based on soft bodied gastropods, such as slugs and nudibranch; in particular, the "blue dragon" sea slug; Goodra's Pokédex entry may be a reference to slug's retractable eyes, which retract when they are in danger.
Alternatively, it could be based on the yōkai called Shussebora, meaning "ascended trumpet shell", which is a trumpet shell snail that lives 1000 years in the mountains, 1000 on the plains, and 1000 at sea, then will ascend and turn into a dragon.
It's probably not Poison type due to it's origins. Goodra seems to be based off of serpents/mollusks, as stated above. Technically, it should be Water/Dragon, due to the mentions of sea slugs and so forth. The most interesting part to me is this:
Alternatively, it could be based on the yōkai called Shussebora, meaning "ascended trumpet shell", which is a trumpet shell snail that lives 1000 years in the mountains, 1000 on the plains, and 1000 at sea, then will ascend and turn into a dragon.
Just a plain Dragon.
As for it learning moves such as Sludge Wave, many Dragon-types can utilize elemental magic both in Pokemon and in myths. Hydreigon learns moves like Surf and Flamethrower, Dragonite learns Hail and Sandstorm, etc.
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