From its earliest days, Rome assimilated different cultures and religions, well beyond those of Greek traditions. There was a particularly strong Etruscan influence, not that the Romans ever wanted to admit it. The most important and best-known god who was inherited from the Etruscans and has no Greek equivalent was Janus, the god of transitions, including beginnings, gates and doorways, and possibly the start of the year.
The traditional story is that Janus gave his name to January, the first month of the modern year. There are at least two problems with this: January wasn’t the original start of the year for the Romans, who like many cultures used to begin the New Year in Spring instead. By the time that more modern names were given to the months, though, it started with Ianuarius, which was supposedly named after Janus. The other problem is that it’s even disputed whether Ianuarius is derived from Janus, or Juno, who in some ancient almanacs was awarded the month instead.
That’s all a great shame, as the association between the start of the year fits so well with Janus, who is classically depicted as a duality, with two faces on a single head, one looking backward (to the old year), and the other forward (to the new): Ianus Bifrons in Latin.
So he appears in Anton Raphael Mengs’ wonderful fresco in the Vatican’s Camera dei Papiri, The Triumph of History over Time (1772). You’ll no doubt recognise Father Time with his long grey beard and scythe in the foreground, behind whom History is busy keeping records. She looks up to the fresh new face of Janus, as the old one looks away to the right.
For all his importance in the ancient city of Rome, Janus has hardly been painted in more modern times. This is because he doesn’t appear in the major references to myths, such as Ovid’s Metamorphoses, which were the sources used by artists from the Renaissance onwards.
He does, though, appear as the occasional statue. In Nicolas Poussin’s brilliant Dance to the Music of Time (c 1634-6), opposite the dancing figure of Pleasure is a small herm of Janus, whose two faces look to the past and the future.
Janus’ association with gates, and the start and end of war, gave rise to an interesting tradition in classical Rome: the gates at each end of an open enclosure associated with the god were kept open in times of war, and closed when the city and empire was at peace. Opening the gates was therefore a mark of starting a war. This is used in two of Peter Paul Rubens’ paintings from late in his career.
In Rubens’ Temple of Janus from 1634, those gates, here imagined to be those of a temple, are being opened to let a warrior through to battle. Above that doorway is a statue of Janus with his two faces.
A few years later, as Europe was nearing the end of the Thirty Years’ War, Rubens was commissioned by Ferdinand de’ Medici to paint The Consequences of War (1637-38). Ares, god of war, is advancing forcefully having just rushed from the temple of Janus, moving from left to right, with his sword bloodied and held low, just as he had painted in that earlier study.
There seem to be few other paintings which refer to Janus in any other way, which seems a great shame.
Wikipedia has an extensive article which goes into detail about the god.