Mephistopheles
Mephistopheles is the un-nervingly intelligent goat adopted by Geoffrey Swivel who becomes his Familiar, life-partner and friend in all times of need in The Shepherd's Crown.
Annotation
The association of goats and the supernatural in Roundworld folklore is well documented. The demon Mephistopheles did not manifest to Faust in caprine form, preferring to look like a kindly elderly family lawyer. But Baphomet, the half-man, half-woman, half-goat, is well attested and even inspired a Tarot card. And the words of Jesus on goats, sheep and their personal qualities are copiously recorded.