I have heard many stories of medieval people suing animals like French people suing rats for eating their crops and the hilarious way in which the rats' appointed lawyer defended them. How a bishop sued caterpillars for eating his vineyard. How a dog won a trial by combat against the killer of his master.
I couldn't believe it so I found the wikipedia article so it seems that this was indeed a thing.
But the problem is that it just doesn't make any sense. They had to know that the accused animals could not understand what was going on, right?
The one of the dog is the only remotely logical one but I think that's an outlier and I couldn't find any source for it
Two different sorts of animal trials took place – civil, and ecclesiastic – and the rationale behind them has generally been explained differently, depending on which the prosecuting authority actually was. I discussed this in some detail in an earlier thread
along with u/cdesmoulins, and you may find the whole thread of interest, but the part most relevant to your enquiry was this:
There were a number of different types of animal trial. E.P. Evans argues in his century-old classic The Criminal Prosecution and Capital Punishment of Animals, following a distinction originally made by the German legal scholar Karl von Amira in the 1890s, that there seem to have been broadly two distinct varieties - secular trials of individual animals such as horses, pigs and cows for committing "crimes" that would have been punishable by law had the perpetrator been human (almost exclusively killing people), and ecclesiastical trials of swarms of "rats, mice, locusts, weevils, and other vermin, in order to prevent them from devouring the crops."
Neither type were show trials, with entirely pre-ordained outcomes. Great effort seems to have been put into ensuring fair play, and animals were defended by human lawyers. Indeed, secular trials, Evans contends, were based on the theory that
'animals, which were in the service of man, could be arrested, tried, convicted and executed, like any other members of his household.'
and were carried out at least in part because it was believed that any homicide, left unpunished, "was supposed to furnish occasion for the intervention of devils, who were thereby enabled to take possession of both persons and places." Evans cites contemporary Catholic doctrine on exorcism in support of this view, though later in his work he also suggests that
'the ancient Greeks held that a murder, whether committed by a man, a beast, or an inanimate object, unless properly expiated, would arouse the furies and bring pestilence upon the land.'
Ecclesiastical trials, on the other hand, had as an underpinning motive an attempt to tame uncontrollable nature and bring it within the purview of a religiously-mandated legal process and, ultimately, divine authority. It was believed that insects and rodents were either instruments of God's wrath, sent for punishment or the sinful, or were sent by Satan, and that, either way, they "were not subject to human control and could not be seized or imprisoned by the civil authorities" but were properly the business of the Church, "whose bishops and other clergy were empowered to... prescribe the penances and propitiations necessary to produce this result." This meant, in turn, that their depredations "demanded the intervention of the Church and the exercise of its supernatural functions," often the exorcism of an entire swarm. Only such "sacerdotal conjuring and cursing" could prevent the ravages of such animals.