Whirlwind

If your opponent has any Benched Pokémon, he or she chooses 1 of them and switches it with the Defending Pokémon. (Do the damage before switching the Pokémon.)

Mega Drain

Remove a number of damage counters from Butterfree equal to half the damage done to the Defending Pokémon (after applying Weakness and Resistance) (rounded up to the nearest 10). If Butterfree has fewer damage counters than that, remove all of them.

In battle, it flaps its wings at high speed to release highly toxic dust into the air.