I was just re-reading the Belgariad series and noticed that Garion's medallion is never described but all the others are. Belgarath has a wolf, Polgara has an owl, and Ce'Nedra has a giant tree. It's been a long time since I've read the Mallorean books, but I don't remember it being described in it either (although if I remember correctly, Durnik gets a medallion with a hammer on it).
I believe it was never defined; possibly a reader could recognize it, but Garion didn't. In Queen of Sorcery you see:
Garion drew out the silver pendant with the strange design on it. The old man took a medallion out from under his own tunic. It was very bright and there was upon it the figure of a standing wolf so lifelike that it looked almost ready to lope away.
Aunt Pol, her one arm still about Garion's shoulders, drew a similar amulet out of her bodice. Upon the disc of her medallion was the figure of an owl. "
Garion reached back and rubbed at his neck where the chain of the curiously carved silver amulet Wolf and Aunt Pol had given him for Erastide had chafed his skin.
But I don't think Eddings ever actually told us what was on it. I've heard Internet Speculation that suggested it was a Moebius strip, but no conclusive answer.