Talyllyn is first seen when Taran sneaks into the Horned King's castle to rescue his pig Hen Wen from the Horned King. Talyllyn spots Taran and barks at him trying to warn one of the guards nearby of the intruder but the mindless guard sneers at him thinking he is "barking at nothing" and drags him away by his chain leash. Talyllyn is seen with the rest of the guards celebrating the capture of Hen Wen. He like the rest of the guards quieted down and he drops his bone when he sees the presence of the Horned King.
Talyllyn is next seen with a different guard chaining Fflewddur Fflam to the dungeon. Talyllyn viciously barks at Fflewddur and is then dragged out of the cell. When Taran, Eilonwy, and Fflewddur are fleeing from the castle, Talyllyn joins the rest of the guards and Creeper to make sure they don't escape. While Taran is protecting Eilonwy and himself with a magic sword from the guards, Talyllyn is retraining Fflewddur by biting on the back of his trousers. Fflewddur manages to break loose from Talyllyn's mouth when the castle gate nearly falls on them. Talyllyn is last seen clinging onto the gate watching the three successfully escape the castle much to his defeat as he falls to the floor. What happens to him afterward is unknown. Possibly, he escaped when the castle is blown up.