A Tradition. The Glatisant or "Questing Beast" started as a tradition played upon young squires: their mentors would take them into the woods to hunt down a beast that has been troubling the local peasants without telling them anything about the creature. The older knight would treat this hunt with all seriousness. After night falls, the mentor would build up the size, speed and ferocity of the creature before describe the beast:
The creature has the spotted body of a leopard, the neck and head of a great serpent, the legs and hooves of a hart, and a long lion's tail. Its footfalls are silent and it can climb a tree as easily as a spider. You can hear the Glatisant call, the barking of thirty hounds hunting prey, right before it ambushes its prey!
Just as the knight finishes the story, the sound of barking hounds come from nearby, and the knight tells the squire to seek out the beast. Of course, there is no beast. The sound of the barking came from a hidden accomplice with several hounds or a dog call. The squire then spends the night, frightened of his own shadow, scrambling around the woods until morning. When they reveal the trick, the squire is embarrassed but gets to borrow the tradition and use it on the next apprentice.
It is all in good fun... until the gods get involved.
A Cosmic Joke Turned Real. Perhaps, a heroic god was displeased with a family of knights or a trickster god wanted to teach a do-gooder a lesson, but the Questing Beasts was made real.
The Glatisant is a large and solitary ambush predator that hunts small birds, lizards and fish. They spend most of their time lounging about in the branches of trees, running at break neck speed that defies their awkward body shape and barking in excitement when they see humanoids. Their call does sound like dozens of dogs yipping. Their natural instinct is to run from humanoids and they seem to enjoy the chase.
They are most vulnerable when they stop running to cool off with a drink.
The Quest. Many knights are tasked with hunting these creatures by the gods but they are difficult to catch. Some families hunt the same beast for generations, since these creatures live for hundreds of years, and the creature is a symbol found on many family crests. While some knights perhaps driven mad by the quest would slay the beast for a trophy but it is considered bad luck to harm one of these creatures. They are not known to attack humans. It is better to release them back into the wild so that future heroes can prove their mettle by catching the Questing Beast. And who knows, perhaps the Questing Beast will reward this act of mercy by coming to the aid of their favorite Questing hunter in their time of need.