Children of the twentieth century know what elephants fear, psychically and culturally if not "in real life": They fear mice. Elephants who see mice jump lightly onto flimsy little wooden stools, which miraculously hold under their great bulk, pull their elephant skins up around their waists like a skirt, and shriek like frightened children. We know this to be their reaction.
But this was not always so! Children of the ancient world knew what elephants feared, with every bit as much certainty as we do today, and what they feared most was the pig.
From Polyaenus' Stratagems:
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.
From Aelian's On The Nature of Animals (emphases mine):
I have stated earlier on that the elephant dreads a pig; I now wish to tell what happened at Megara when the Megarians were besieged by Antigonus, and the story I have to tell is as follows. When the Macedonians were pressing them hard, they smeared some pigs with liquid pitch, set a light to them, and let them loose against the enemy. Goaded with pain and shrieking because of their burns, the pigs fell upon the troops of elephants, driving them mad and throwing them into terrible confusion. So the elephants broke ranks and were no longer tractable in spite of having been trained since they were small, either because elephants by some instinct hate and loathe pigs, or because they dread the shrill and discordant sound of their voices. In consequence those who train young elephants, being aware of this, keep pigs along with them, so it is said, in order that through herding together the elephants may get to fear them less.
I hate to quibble with either Aelian or Polyaenus, but I'm afraid I must. The lesson one should draw from the siege of Megara, where a group of war elephants fled from a herd of incendiary war pigs, is not "Oh, elephants must be afraid of pigs, probably because of their unusual high-pitched voices; to combat this, let's make sure to raise all our future elephants with a pig friend," but that animals do not want to be on fire, and will run away from it given the opportunity. If this makes me a military historian, so be it; I will accept the responsibilities and consequences that come my way as a result.
[Image via Wikimedia Commons]