I've encountered some pretty narky sheep from time to time. Once got chased over a stile by a couple of rams on Fountains Fell in N Yorkshire.

There's a strange breed of sheep in Ayrshire that are quite big n' hefty with wide, bullish faces that look like a bad idea to tangle with too. There was one that used to lay in the middle of the road near Lochwinnoch and wouldn't even move for cars. Looked like a right hard f*cker!!