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Why was Dark-type special until the split?
Every damaging Dark-type move up through Gen II ended up becoming a Physical move, and they all sounded Physical as well (Bite, Crunch, Thief, Beat Up, Feint Attack.) Bite even WAS a Physical move in Gen I before becoming Dark.
Every Gen II Dark-type has better Attack than Sp.Atk, except Houndoom (who's Atk is still good) and Murkrow (who's offense stats are equal.) Even Umbreon's Attack is 5 points higher than its Special Attack.
Yet, Ghost was Physical... but every Ghost type through Gen II was a Special Attacker. Hell, both of Gengar's STABs were Physical types.