Cerridwen, technically a goddess, but Christians used her as their model for what they thought a witch was, especially the cauldron.
Fom
www.celticpagan.com :
Cerridwen, a Celtic Goddess known for creating a magical brew that was stirred for a year and a day to produce the Three Drops of Inspiration. These were stolen by a young boy who grew up to be known as Taliesan, a great bard and wizard. The myth of the Goddess' pursuit of this thief involves a dramatic series of shape changing. Cerridwen is the muse who brings inspiration to poets, musicians, writers and those of the creative arts. Historically, Celtic bards were known as Credo, children of Cerridwen.