It is semi-sentient, able to move under its own power and seeming to possess a degree of intelligence. It is also shown as being rather small, almost the size of Eilonwy's head. It loves to chase rats, and is afraid of witches. It disappears in the middle of the film and reappears in Morva. It is not seen when the Horned King raises the dead, at the sacrifice of Gurgi and the resuscitation of the latter. It is last seen skipping along home to Caer Dallben with Taran, Fflewddur Fflam and Gurgi at the end of the film.
In the film, the Horned King believed it could find The Black Cauldron and kidnaps it and Eilonwy. In the lower ruins under the Horned King's castle, Taran meets Eilonwy and the Golden Pelydryn and is surprised to see it. He touches it and finds that it illuminates and Eilonwy explains that it is a magic bauble. Eilonwy then explains the reasoning for their imprisonment, while Pelydryn chases the rats in the ruins. This is the only scene where it's personality is developed. While exploring the lower ruins, Taran and Eilonwy come across the mystic sword and free Fflewddur Fflam from his imprisonment. During those two events, it appears light blue because of the dark surroundings. The Golden Pelydryn is the first to see Fflewddur Fflam in his solitary confinement. Afterwards, the Pelydryn is not seen until the scene where Gurgi indicates the whereabouts of the "wicked cauldron" and it later hides when the Witches of Morva discover that they don't have anymore transfigured frogs. Afterwards, Pelydryn is not seen again until the end of the film.
In the novels of The Chronicles of Prydain, the Golden Pelydryn is rather small, roughly the size of a baseball or similar, and incapable of moving on its own, Eilonwy carries it at all times. It is also unable to light on its own, as the movie indicates it can do. In the film, Eilonwy's bubble is never identified by its proper name; this is understandable, as the name of the Golden Pelydryn was not known to the cast until the third book.