To the right is what I call the "Herakleia" plate. In Anders Bergquist, & Timothy Taylor, The origin of the Gundestrup cauldron Antiquity 61, 1987, p.10-24, the authors describe the device on the left thus:
"To the left of one of the female busts, a full-figure man wrestles a vicious animal and a flower blooms beneath them"They then go on to describe a silver vase where a female face is flanked with figures of men. They do not mention the figure to the right at all.
In Flemming Kaul, The Gundestrup cauldron reconsidered, Acta Archaeologica vol 66, 1995, pp 1-38, this plate design is not mentioned at all, but there are various comparisons of isolated details of the cauldron to other objects.
Greek coin of Herakleia, Lucania, Italy showing the head
of Athena wearing a helmet decorated with Skylla
hurling a stone and Herakles wrestling the Nemean lion
Image courtesy of Classical Numismatic Group Inc.
Pyrrhus had Celtic troops with him in Italy, and it was his terrible losses at the Battle of Herakleia that gave us the term "Pyrrhic Victory". Although history does not record these Celtic troops, the earliest coin of the Belgae is one of the Ambiani,and is copied from a gold stater of Taras, which Pyrrhus also tried to defend against the Romans. The Ambiani thus were among Pyrrhus forces (the gold is highly refined Mediterranean gold, and the Celts derived their coinage designs, at first, from money paid to them in the Italian campaigns) On another Gundestrup plate is the figure of a man on a dolphin. This man is Taras, who is depicted thus on most of the staters of Taras and is not Arion (the latter has no context on the cauldron). Pyrrhus' elephants (not recognized as such) are also depicted on the cauldron in association with the same female bust.
The woman, who is a Celtic Persephone (identified from another Gundestrup plate), beats her chest in an attitude of grief, as she does on the elephant plate and the figure on the right bears a strong resemblance to the "mannikins" seen on the earliest coins of the Treveri and subsequently on the coins of the Aulerci Cenomani where it is winged, and on the coins of the Namnetes where it sometimes grasps a horse's leg with each outstretched hand. The general posture of this figure is also matched on another Gundestrup plate depicting a "wheel-turner". A pair of such wheel-turners (in a different stance) are depicted on a sword from Hallstatt (Jacobsthal, 96). This plate commemorates the loss of warriors of a specific tribe who fought with Pyrrhus at Herakleia.