The 16th century was seeing warfare get revolutionized. Gunpowder had shown up on the scene and changed warfare from the way humans had been accustomed to fighting forever. With every drastic change, come new strategies, and a good number of them are likely to be wacky; like these illustrations from a 1530 war manual that advocates using animals to deliver weapon payloads.
Recently identified illustrations show directions from artillery master Franz Helm of Cologne on the use of cats and birds in warfare. The idea in the manual encouraged commanders to capture a bird or cat from the castle under siege. An inflammable small sack or a fire arrow would be strapped on to the feline or bird, and they would be let loose. According to the idea, the scared animal would run straight home, and hopefully reach home just in time for the explosion to start fire in the target castle.
It appears that the technique was not used. Though several analysts do say the chance of the animal returning to the castle in time for the explosion was quite difficult. The army using this technique would also run a significant risk of having the animal set fire to their own camp instead.