The peacock is an image of heretics and fanatical spirits. For on the order of the peacock they, too, show themselves and strut about in their gifts, which never are outstanding. But if they could see their feet, that is the foundation of their doctrine, they would be stricken with terror, lower their crests, and humble themselves. To be sure, they, too, suffer from jealousy, because they cannot bear honest and true teachers. They want to be the whole show and want to put up with no one next to them. And they are immeasurably envious, as peacocks are. Finally, they have a raucous and unpleasant voice, that is, their doctrine is bitter and sad for afflicted and godly minds; for it casts consciences down more than it lifts them up and strengthens them.”
Martin Luther, What Luther Says, An Anthology, 3 vols., ed., Ewald Plass, St. Louis: Concordia Publishing House, 1959, II, p. 642.