Imagine trying to prosecute someone who could only respond with oinks and grunts. Believe it or not, trials of this type did happen many times in the middle ages and continue to take place in non-Westernized countries. Mental Floss has a great collection of these stories, including a pig who was executed in people clothes, a donkey who escaped a bestiality charge through a number of character witness testimonies, and a lawyer who was able to help fight off the charges against every rat in Autun, France.
If that's not enough animal tails trials for you, then check out Crack's article on the same subject that includes some of the same stories, but also a few different court cases including a cat fighting for its right to free speech and a chicken who was charged with laying an egg without a yolk -which could obviously result in a basilisk baby if it was hatched. In case your wondering, both of the animals named lost and while the cat simply had to stop meowing on the sidewalk for money, the chicken was burned alive on the stake with her egg.
Of course, animals have been known to be on the other end of proceedings as well. There's always Tabby Sal, the cat who was called for jury duty. While the issue was eventually resolved and the cat didn't end up serving on a case, I can only imagine what kind of horrible revenge would be in store for us if animals did start to serve on juries.