Furfur

In demonology, Furfur (other spelling: Furtur, Ferthur) is a powerful Great Earl of Hell, being the ruler of twenty-six legions of demons.

He is a liar even when compelled to enter a magic triangle, where he answers questions, speaking with a rough voice.

Furfur causes love between a man and a woman, creates storms, tempests, thunder, lightning, and teaches on secret and divine things.

To some authors he changes from deer into angel when compelled to enter the magic triangle.

[1] However it seems more likely that the name is a corruption of 'furcifer', the Latin word for scoundrel.