How the Vikings captured a city in Sicily

"Now when Harald came to Sicily he harried there and with his army laid siege to a great and populous fortified city. He surrounded the place, because it had strong walls, so that it seemed unlikely that he could break them down. The townspeople had sufficient victuals and other things required to resist a siege. Then Harald hit upon this stratagem: he let his fowlers catch little birds which had their nests in the city and tie plane shavings of resinous pine soaked with molten wax and sulphur on their backs, to which he set fire. When liberated, all the birds at once flew into the city to seek their young and the nests they had under the thatches of reed or straw. And then the fire spread from the birds to the house-thatches; and though each single one carried but little fire, it soon grew to a conflagration, since many birds carried it all about the thatches of the city; and soon one house after the other began to burn till the whole city was aflame. Then all the people came out of the city and begged for mercy—the very same who many a day had spoken overbearingly and scornfully about the Greek army and its generals. Harald gave all those quarter who asked for it, and made himself master of that city." 

--- excerpt from Chapter 6 of the "Saga of Harald Sigurtharson," in Heimskingla, by Snorri Sturluson.

I have heard of other legendary warriors using animals carrying flames to set a city on fire - most notably Genghis Khan, who...

"...used hapless animals in the Mongol campaign against the Tangut kingdom of Hsia Hsi in 1207. In a feint he offered to end the siege of the well fortified city of Volohai if he was paid a “tribute of one thousand cats and one thousand swallows”. Genghis Khan then set them alight and released them in one great rush of living fire. The wretched animals set the city alight in hundreds of places simultaneously. While the defenders fought the flames, the Mongols breached the walls of Volohai."

The Sicily incident recorded in the Heimskringla was written by a 12th-century historian, reporting on the conquests of Harald Hardrada in the 11th century - so fully two centuries before Genghis Khan.

A quick search reveals a similar technique in the early Song Dynasty (960-1279):

According to Pr. Shi Bo, in "Trente-six Stratagèmes Chinois", monkeys were used in the beginning of the Southern Song Dynasty, in a battle between rebels of the Yanzhou (Yasuo) province and the Chinese Imperial Army, led by Zhao Yu. The monkeys were used as live incendiary devices. The animals were clothed with straw, dipped in oil and set on fire. They were set loose into the enemy's camp, thereby setting the tents on fire, and driving the whole camp into chaos.

Addendum:  Reader Galileo Feynman reminds us that the United States attempted to use "incendiary bats" against Japan during WWII (discussed in this NPR podcast).