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My theory is that this Pokemon is the reason Nail later got turned into Curse. They figured the "you have to get it to level 100 to make it do the thing it's made to do" idea was stupid, so they reworked its signature move to have different effects. Instead, they changed how the move worked and changed its evolution's typing (dark? normal?). Thus, it only works as a Curse-staller (it learns Pain Split, Sub, and Night Shade all through levelup) if you don't evolve it and take away the ghost typing. Once you evolve it, its signature move turns it into a tank, which seems to go along with the evolution learning more physical moves.
But then the whole line got scrapped anyway and Curse was left as an oddity for no explained reason.
