Hercules was the Romanised version of the Greek deity Heracles. A man of great strength: a demi god born in Thebes of Jupiter and a mortal woman. It is possible that the cult of Hercules replaced another similar cult of local Roman/central Italian origin called Semo Sancus. Semo Sancus was son of Iupiter and god of oaths, marriage, hospitality, law and contracts: like Hercules he was avenger of and guardian of oaths.
Roman mythology generally ascribed the introduction of the cult of Hercules into Italy and Rome to the Greek Evander, who settled in central Italy and founded the “city” of Pallantium on a hill near the Tiber river at about the time of the Trojan war (ie long before the founding of Rome). When Rome was founded the city of Pallantium was integrated into it, as was another city supposedly founded by Hercules, on the neighboring Capitol hill.
Virgil’s Aenid suggests that it was in fact Evander who erected the important great altar to Hercules in the Forum Boarium on the site where he slew the giant Cacus. This altar was known as the “Ara Maxima” or “Ara Magna” dedicated to Hercules Invicti – Hercules unbeaten and it predated the later round temple to Hercules Victor, also at the Forum Boarium. The altar burned down in the great fire of Rome (yes, the one where Emperor Nero supposedly played his lyre whilst watching it burn) but was rebuilt and many centuries later demolished by the Catholic church. Recent research has found a tufa rock platform which may have been the base of the Ara Maxima, in the church of Santa Maria in Cosmedin (across the street from the round temple to Hercules Victor).
From time immemorial until the end of the 3rd century BC the temple and altar to Hercules were a private domain of the gens Pinaria (the Pinaria family). Around the year 295BC Appius Claudius made the cult of Hercules public and in so doing also opened the path so that the tithes paid to Hercules’ estate might go straight into the state coffers.
Invoking Hercules was a common expression in Latin – rather like someone 50 years ago might have exclaimed “by Jove!” so a Roman would have said “by Hercules!”.
Hercules was greatly revered for his strength and courage and as such stood as an example and patron to individuals such as Mark Anthony – his soldiers interpreted a variety of signs on the eve of the battle of Action as a sign that Mark Anthony no longer had the protection of Hercules.
Hercules was also the protecting patron of ancient roman gladiators (for obvious reasons). Should a gladiator reach the end of his (or her!) career it was customary for him to be awarded a wooden sword which would be hung in the temple of Hercules, which still stands by the river Tiber in Rome.
His association with physical prowess was extended during the imperial age – as emperors maintained social stability through “bread and circus games” so Hercules came to be patron of the athletic games which they revived for common popular consumption, and in particular he was made patron of the athlete’s guild composed of professional athletes who toured the empire.
The mythology around Hercule’s strength and perhaps his essence as half man-half deity made him a recurring theme in later heraldry.