The idea of flaming pigs, to my knowledge, was mainly made popular by the Alexander Romance, a medieval collection of stories about Alexander that was widely read. One of the earliest versions of the Alexander Romance, Pseudo-Callisthenes (which was first written down some time in late antiquity), tells the story that Alexander learned of the tactic against Porus. Of course, the Alexander Romance is no more factual than the Arthurian Romance, and often is a lot more ridiculous--no source attests to Alexander's used of pigs. The most important use of pigs against elephants, and indeed the more famous of two that actually survives from antiquity, is Aelian's claim that Pyrrhus' elephants failed to capture the Roman camp after defeating their army because they were frightened of the squealing of the pigs that the Romans had brought for food. Interestingly some of the earliest Roman coins, dating from soon after the Pyrrhic War, feature images of boars, but it's likely not to bee related. The only instance of flaming pigs, and the second of only two references from antiquity to pigs on the battlefield, is their use during thee siege of Megara by Antigonus Gonatus. According to Aelian (guess he like pigs) and a minor Greek sourced the Megarians broke the siege by lighting pigs smeared with pitch on fire and using them to throw Antigonus' elephants in panic.
Those are the only references to pigs in battle during antiquity, although I think there may be a couple more in the Middle Ages
FYI, there was a response about war pigs in this thread from FAQ section Use of animals in battle:
and a few minor (and conflicting) mentions in these