Hm, I can clarify that it does NOT need to thunderstorm on the first Zangoose encounter. I just encountered it myself, on a windy night. Not sure if that's the necessary weather, but it was not thunderstorm.
Also, Electrike's encounter MUST be thunderstorm weather.
Hopefully this clears up some misunderstandings anyone might have had.
EDIT: Well, I am stupid. Did not realize I posted on this thread earlier today. Sorry!
Well, I guess it's helpful for Qaaz so he can see the thread has a new post...