This page from a 1584 treatise on explosive weaponry reveals two of the 16th century's more mobile methods of destruction: the cat bomb and the bird bomb, apparently used to set buildings on fire. BibliOdyssey has more fiery weapons from this manuscript, the rest of which are safer for friends of felines.
During WW-II, FDR personally approved development of a bat-bomb to be used against Japan. Each bomb would carry hundreds of bats equipped with timed incendiary charges. Released over a Japanse city during daylight hours, the bombs would open at 1,000 feet, releasing the bats. The bats would then fly down to roost under the eves of houses, which were made mostly of wood and paper, easily set ablaze when the charge went off. Prototypes were built and tested but the weapon was never deployed; the project was cancelled when it became clear that delays in developing the release mechanism would prevent the weapon from influencing the outcome of the war. The only damage the bats ever caused was to an Army base when some armed bats escaped, roosted under a fuel tank and set the whole place on fire.