LogFAQs > #978745932

I think the reasoning for Ghost being Physical back in the day was that it was intended to be a counter to the Psychic type, which always had a huge Special stat. This was kind of a pointless idea though since prior to GSC the only non-fixed-damage Ghost move was Lick, which was 20 base power, thus absolutely useless for any other purpose than trying to get a paralyze in. And by the time better Ghost moves were added like Shadow Ball, the Special stat had been split anyway.

No idea what the rationale was for making Dark a Special type, though, since literally all of the moves were physical in theme, the two biggest top-of-my-head examples being Bite and Crunch. Just didn't make any sense.