Arawn was an evil overlord in the Chronicles of Prydain books by Lloydd Alexander. He was replaced by the Horned King in The Black Cauldron because the animators felt that people would like the Horned King better since he had horns (similar to Maleficent and Chernabog). However, Arawn did receive a part in the movie. During the prologue, the narrator talks about the history of the Black Cauldron and how Arawn came to become it. Arawn is the minor antagonist of the film Black Cauldron.
The Black Cauldron
As stated in the prologue: "Legend has it, in the mystic land of Prydain, there was once a king so cruel and so evil, that even the gods feared him. Since no prison could hold him, he was thrown alive into a crucible of molten iron. There, his demonic spirit was captured in the form of a great Black Cauldron."
Arawn's face is apparently engraved upon the cauldron. At certain points in the movie, the face is illuminated. There are several signs that the Horned King once served under Arawn as he says that now the power shall return to this world. Another sign of his connection to Arawn is that while being sucked into the Black Cauldron, he shouts, "Nooooo, Master!"
House of Mouse
In the House of mouse episode, House of Magic, Daisy Duck accidentally makes the House of Mouse audiance disappear. When they ask the Magic Mirror for help, he tells them to check the prop basement for something magical to help them. Daisy and Minnie find the Black Cauldron (a sign on it says: Who So-Ever Uses the Black Cauldron for Evil Will be All Powerful), and deside it's what they need.
Daisy dresses as Maleficent and attempts to cast a spell. She ends up summoning the Cauldron Born instead. In order to make it go away, she casts a spell. However, in the process, she makes the whole House of Mouse disappear. After everything was set straight by Jafar, Daisy attempts to use the Cauldron as a fondu pot, again summoning the Cauldron Born.
Behind the scenes
Arawn was originally to be the main villain of the Black Cauldron, just like in the books. However, Arawn was replaced by the Horned King because animators felt that audiances would like Horned King better because he had horns. Several people working on the film were discouraged about this because they had read the books and felt that you couldn't do the film based on the book without the book's main villain. The film workers urged the producers to give Arawn a part in the movie so they made him a spirit trapped in the cauldron.