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I saw that there was a grub that just evolved into... another grub, and that kind of got the gears in my head turning. You see this simple-looking, cutesy grub Pokemon, train it up, it evolves early as usual, and then turns into... a bigger, fatter grub without much flair.
As far as the Dark typing goes, I kind of had an edgy idea for its dex entry/lore. Basically these cute lil' things are born in huge batches like most insects, but were never meant to evolve. However, as their environment grew more competitive, and their itty-bitty size wasn't cutting it, the species took an unorthodox route to unlock the potential needed to stave off extinction.
Mass fratricide, to the point where most members of the species devour every single one of their siblings shortly after birth, giving them the strength to evolve into this obese, horrifying and amoral beast of a bug.