The Flaming Pigs of Megara

Greek military writers believed that war elephants were frightened of pigs, and they tell us that various Roman and Greek commanders tried to break up elephant attacks by releasing pigs or wild boars among them. When the Hellenistic ruler Antigonus laid siege to Megara in 266 BC, the Megarans went one better on this:

At the siege of Megara, Antigonus brought his elephants into the attack; but the Megarians daubed some swine with pitch, set fire to it, and let them loose among the elephants. The pigs grunted and shrieked under the torture of the fire, and sprang forwards as hard as they could among the elephants, who broke their ranks in confusion and fright, and ran off in different directions. From this time onwards, Antigonus ordered the Indians, when they trained up their elephants, to bring up swine among them; so that the elephants might thus become accustomed to the sight of them, and to their noise.

–Polyaenus: Stratagems, IV, 6

All accounts of flaming war pigs go back to this one source, so whether this actually happened is as unclear as with catapulting pots full of poisonous snakes. For some thoughts on how ancient Greek and Roman armies actually neutralized elephants, see here.