This may be more of a misconception than a stereotype, but I think it can still fall under the “stereotypes” category, especially for some non-Pagan groups who do think that all Pagans (or most Pagans) worship or believe in Satan.
This is simply not true. Most Pagans don’t even believe in Satan or the devil. In order to worship something, you need to believe that it is true. If one does not believe in the existence of the devil, then how can one worship him? The devil/Satan is a Judeo-Christian concept, and while there are dark gods and goddesses in any world mythology a Pagan may choose from, these darker-aspected deities do not automatically equate with Satan. Satan is a strictly Judeo-Christian personage, and to say that Pagans worship Satan is very much incorrect.
I’m sure by now someone has thought of Satanists. Yes, there are Satanists, and yes, as far as I can tell (with my very limited knowledge, mind you), they do worship Satan. However, the majority of Satanists don’t even consider themselves Pagan, and most Pagans do not lump Satanists under the Paganism umbrella.
I’m sure this misunderstanding and stereotype arose over many hundreds of years after cultural and societal beliefs began to connect the darker Pagan gods with the Judeo-Christian Satan, especially those deities who dealt with death. The image of the devil is actually connected to images of Pan and the Horned One, so it’s not a surprise that if culturally the images were changed to match the idea of Satan, then Pagans of all types would be equated with Satan worshipers.
I do believe it’s less common nowadays to hear “so if you’re Pagan, do you worship Satan?” than it used to be. I have, however, had that question posed to me on a number of occasions, so I think it still can be counted as a stereotype. After all, I’m not exactly fully out of the closet, so I can’t be the only one who’s gotten that question!