Gengar - While it is more popular on ghost and with its loss of levitate in gen 7, gengar isn't seen a ton on standard poison teams. However its ghost typing and speed tier allow it to help immensely with opposing psychic types like Mega-gallade, and can act as a greater form of speed control than nihilego.
Golbat - While it may appear to be inferior to crobat in every way, Golbat is actually bulkier than crobat with an eviolite, and can almost bring the same amount of utility crobat had. With it only losing a ton of speed, with a proper team golbat and its added bulk allow poison's already strong defensive core even more strong.