There are several historical examples in our world of animals being arrested, excommunicated or killed for various crimes. Articles in the October 1994 issue of Scientific American and in The Book of Lists #3 give several examples: a chimpanzee was convicted in Indiana in 1905 of smoking in public; 75 pigeons were executed in 1963 in Tripoli for ferrying stolen money across the Mediterranean; and in 1916, “five-ton Mary” the elephant killed her trainer and was subsequently sentenced to death by hanging — a sentence that involved a 100-ton derrick and a steam shovel. But the law is fair, and sometimes the animals get the better of it: when in 1713 a Franciscan monastery brought the termites who had been infesting their buildings to trial, a Brazilian court ruled that termites had a valid prior claim to the land, and ordered the monks to give the termites their own plot.
Note that Soul Cake Thursday in later Discworld novels becomes Soul Cake Tuesday, after previously having been Soul Cake Friday in The Dark Side of the Sun.