Hitherto Unknown Names; & Issues with Scale
As far as I can tell, by and large, what this art-form's contribution has been is pretty much just the addition to the mythology of a handful of stories and personages unattested in literature, e.g. there are a few Gigantes who fight against the gods and whose names do not occur in any currently extant ancient mythography. These are Pancrates, Euphorbus, Euboeus and (what has to be the coolest name among the Giants) Uranium.✭ For the most part the trickiest thing about virtually every ancient Greek art-form (particularly sculpture and pottery) and its correlation to the written stories also has to do with the Gigantes, who, at least in comparison to the gods and to humans, to do not seem to have been gigantic at all!
The vases depicting Heracles' fight with the Libyan Antaeus generally have the two wrestling opponents at the same size. The same goes for the Gigantes who attack the town of the Olympians in what comes to be known as the Gigantomachy: they are the same sizes as their enemies, with the exception of Alcyoneus, who is usually huge in comparison to those who contend with him. The Cyclops Polyphemus is also usually about twice the size of Odysseus.
Perhaps the easiest way out of this conundrum is to conclude that the painters simply didn't have enough space in which to work for the sake of portraying these characters to scale. Otherwise one might have to say that the Gigantes (and also the Titans, who, on pottery, generally are also simply the same size as deities and human beings) only gained their great stature later in writing.
Snapshots of Tales Untold
Heracles vs Geras on Attic red-figured pelike [Photo Taken May 6, 2013, by Egisto Sani]
There is a lost myth in which the great hero Heracles, in some context, fights against Geras, "Old Age" himself, presumably winning in the end(?). Perhaps the sequel to this (or vice versa) is what we do know from writings about Heracles' ultimate fate, in which he marries his own half-sister the goddess Hebe, "Youth," after gaining immortality for himself. Heracles had, appropriately, also fought against and defeated Geras' brother Thanatos, "Death." According to Wikipedia, Geras is primarily known from the vase paintings depicting him with Heracles.✭✭
Speaking of lost myths, it is most incredible to me that there might be missing stories, floating out there in the aether, about winged Centaurs!
How could it be possible that such cool creatures don't receive a single mention in any major (or even minor) myth, especially considering how prominent Centaurs are during Heracles' lifetime? Well, this being yet more "lost mythology," I suppose these creatures may have appeared at any other random point in the chronology, far removed from the more famous characters.
A More Bovine Nessus?
And speaking of Heracles and Centaurs, here (above) the Centaur Nessus appears being manhandled by Heracles. The Centaur's lower features distinctively seem to make him into, rather than the more mainstream Hippocentaur ("Horse-Centaur") variety of his kind, into a Bucentaur, a creature with the exact same configuration as an equine Centaur, except with bull rather than horse parts.
✭ Well, actually, in the original Greek Oranion or Ouranion ("Small Heaven"?).
✭✭ That image, though, depending on how you take it, perhaps frames Heracles in a really bad light, since he's really about to beat up on a little old man with his huge club.