All I could find it this:
Scorpio, the eighth sign of the zodiac, is traditionally represented as a scorpion, but sometimes it is symbolized by a very different creature, the eagle (occasionally the phoenix). Why might this be so? Christian tradition links the four Evangelists (the attributed authors of the canonical Gospels) with four symbols: Matthew with a Man or Cherub, Mark with a Lion, Luke with a Bull and John with an Eagle. These symbols are derived from Ezekiel's famous vision in which he saw four creatures, each with four faces, as described in Ezekiel 1:10:
Their faces looked like this: Each of the four had the face of a man, and on the right side each had the face of a lion, and on the left the face of an ox; each also had the face of an eagle.
Parts of this vision are rehashed by John in Revelation. In the fourth chapter, four creatures are again encountered, and their appearance is described in Revelation 4
The first living creature was like a lion, the second was like an ox, the third had a face like a man, the fourth was like a flying eagle.
http://www.byzant.com/Mystical/Astrology/Eagle.aspx (read more here if you'd like)
It's still a bit confusing for me.