There was an item in the news a few months ago about a herd of cows gone bad, which could be the basis for a surprise encounter.
The story is that the farmer had been prosecuted for keeping the cows in a poor condition, and (I think) was spending a spell in prison for said offence. Meanwhile, the cows, already rather shaggy and diseased, fell between the jurisdiction of several agencies. Neither the police or the RSPCA (UK equivalent to NSPCA, i.e it's 'Royal' rather than 'National') could take charge of the cattle because of some obscure property laws. So the cows remained in a field, slowly going feral. All the authorities could do was to occasionally shoot one of the more distressed animals from a distance.
Point being, though, that one thinks of domestic cattle as being large, but generally harmless, but these cows, due to neglect and a lack of human contact, had become violent and dangerous, and that's not just the bull.
It's quite typical in FRP to come across an abandoned farm (struck by bandits, goblins etc.). You can bet that the PCs wouldn't be expecting to be attacked by feral cattle...