Taken from source

A new pokemon was found recently. No one could really identify the creature or even get close due to it's potent stink.This species seems to be invasive to this region and is causing great disturbence in the local pokemon life,completely ruining an entire area of the forest.
Pokeballs don't seem effective and no one can stand it's horrible stench.
The pokemon seems to have captured a Pichu to pleasure itself,also acting as a plug to hold her stink in.
Looks like the problem solved itself, because really no one wants to deal with that. We are thankful for your sacrifice pichu.