Cats

 While cats in the middle ages were looked upon with suspicion, their value in keeping the rodent problem down was not unnoticed. Many monasteries kept cats for this very reason. Books were painstaking labours of love, written and illustrated completely by hand. Rats would often chew on pages or tear them out completely to make nests; thus, monks kept cats.

But cats, being cats, came with their own set of problems. One example comes from a 1420 manuscript that details one such issue. Two pages of the manuscript have been left empty, save for a drawing of a cat and accusatory fingers pointing to an explanation scribbled (probably angrily) alongside the picture. Written in Latin, a rough translation reads: “Here is nothing missing, but a cat urinated on this during a certain night. Cursed be the pesty cat that urinated over this book during the night in Deventer and because of it many others [other cats] too. And beware well not to leave open books at night where cats can come.”
So cats really haven’t changed much over the centuries.

Source:
Historisches Archiv, Cologne