The Questing Beast—or the Beast Glatisant, comes to us from Arthurian legend. It’s physical description is usually something of a chimera—head of a snake, body of a leopard, haunches of a lion, and feet of a deer. Confused? Well, so is it.
The legend inside the legend (revealed by Merlin) is that the Questing Beast was born of a human woman—she fell in love with her own brother, and slept with a devil who promised to make the brother love her in return, but instead the devil tricked her into accusing the brother of rape—which resulted in her father ordering him to be torn apart by rabid dogs. As he was dying, he cursed her, saying she’d give birth to a hideous creature, which would make the same sounds as the animals that killed him—which is where Glatisant comes from, the French word for “yelping” or “barking,” glapissant.
The Questing Beast is sort of a symbol of incest, destruction, and all kinds of bad things rolled into one. In most legends, Arthur first sees the Questing Beast after sleeping with his (unbeknowst to him) sister, Morgause, who bears Mordred, who ultimately becomes Arthur’s downfall. In most legends it is also understood that it is the family quest of the Pellinore line to hunt the Glatisant—thus the “Questing” Beast. T.H. White makes a running joke of this in The Once and Future King, using Pellinore and also a few other knights, including Sir Palomides, who is also linked to hunting the Questing Beast, in later stories.
There is another, completely different version of the Questing Beast, however. The Perlesvaus represents the beast as beautiful, pure white, and smaller than a fox. likening the beast to the church of Christ. This version seems to have an entirely different background, and it’s “barking” sounds are accounted for by being the creature’s offspring, ripping it to shreds from the inside—representative of impious churchgoers. The first, more traditional representation of the beast is understandably more popular. Its symbolism of destruction is befitting of the fall of Arthur, also.
The best place to go looking for the Questing Beast, then, is of course in the megaliths of Arthurian legend themselves, like Le Morte d’Arthur by Thomas Malory, and The Once and Future King by T.H. White.