I cannot recommend that book strongly enough. The Criminal Prosecution and Capital Punishment of Animals is one of the most unusual, provocative and interesting books I have ever read. From my reading, I did not recall that they were necessarily commonplace but such trials did in fact exist and there were enough of them to fill a book. In the author's analysis, he also says that it's important not to mistake these trials as reflecting any sort of mass stupidity or gullibility on behalf of the population or clergy, but rather they indicate the power and control the church held and its esteem in everyday life. Unexplainable things needed explaining, and often blaming animals--or anthropomorphizing them--was the easiest possible explanation. Why are we having this plague? How can it end? Let's banish some locust and everyone will feel better! It was simply a way of reinforcing and reasserting religious power, regardless of impact.