Black Bull Stickers

[R60EST]

I keep seeing cars with a Black Bull sticker on the back. Anyone know what they are supposed to represent ?

[R60EST]

After spotting a few a while back I assumed they were some sort of tourist / souvenir thing but in the last few weeks I've seen loads more and thought there was some hidden meaning behind it or some sort of club / forum logo. It was seeing one on an L Reg Nissan Micra that really got me curious because prior to that I'd only seen them on sports / prestige cars