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I think Necturna would be a pokémon in category with Lucario and Zoroark.
A special pokémon though not legendary and with one pre-evo either hard to get or event-only.
Either way I don't think we could make two pre-evos though if that's how it turns out, we could find out something...
I agree with the comments about the BST simply being too high not to have a prevo, and like others have said, most plants and ghosts go through some sort of evolution. Plus, just look at the thing. THAT should NOT hatch from an egg.