I think it's our local bicycle club, because usually it's in the packs of the road riders wearing silly clothes.
You see them ahead, riding single file along the edge of the road. Just as you start to overtake them, they fan out across the roadway, forcing you to swerve and slam on the brakes. They will stay in this formation until you get around them, and then go back to riding single file.
Yesterday it was a solo rider. As I started to overtake him he swerved all the way across the road into opposing traffic. He wasn't content with just making me slam on the brakes and swerve, he had the car coming the other way doing the same. Lucky for him I expected it and the other driver was paying attention. So we didn't hit each other, and we didn't hit him. Well, at least he seemed startled, I think he was so busy focusing on me he forgot about cars coming the other way.
One of these days, a rider or two is going to be converted into bloody rags.