The vision of the Four Beasts shows transformation just like the metals of the Colossus [Nebuchadnezzar's statue] dream.
The descent is from the Lion, the King of Beasts, to a nondescript Monster that has no description.
The reason these Four Kingdoms are represented first as a Golden Headed Metallic Image and then as a succession of Wild Beasts is to show the difference between man's view and God's view of the World Kingdoms.