This is a piece from Commentary on the Apocalypse by Saint Beatus of Liébana. He lived from c.730 to c.800, and this work was done in 776. He was a monk, theologian and geographer from Spain. The Commentary on the Apocalypse is a 12-volume work on the book of Revelation and the writings of the Old Testament prophet, Daniel, done on parchment. It was found in the Saint-Server Abbey. This work was used as treasured by medieval Christians and many copies were made. The bird here represents Christ, and the snake represents Satan; Christ is victorious over Satan. And so, the depiction is of Christ defeating the Devil.
Image Credit: http://bibliodyssey.blogspot.ca/2010/10/beatus-apocalypse.html