When most people think of monks, they think of unworldly ascetics leading an austere life of prayer, hard work, and devotion. But this isn't the life that Chaucer's monk leads. He's a representative of a corrupt and worldly church whose servants are at times greedy, slothful, and driven by all manner of lusts and desires.
The monk in "The General Prologue" doesn't spend all his time cooped up inside a monastery, saying prayers and peeling potatoes. He'd much rather prefer to be out in the great outdoors, riding horses—as he's doing here—and hunting hares. And he's prepared to pay a lot of money for the privilege:
Of priking and of hunting for the hare was al his lust, for no cost wolde he spare.
Moreover, the monk, like his fellow pilgrim the Prioress, is far too elegantly and expensively dressed for someone who's supposed to be a poor, humble servant of God. Monks normally wear plain habits with hoods, which affirms their devotion to a life of poverty—but not this monk. He has gray fur on the sleeves of his cope and wears a gold pin with a love knot on the end of his hood. The gold is symbolic because it shows where this monk's true devotion really lies.