Another approach mentioned in Let My Nation Serve Me by Rabbi Deutsch (published by Artscroll) relates to astrology. For those unfamiliar with the twelve zodiac signs, they are Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces. In order, they are the sheep, bull, twins, crab, lion, woman, scales, scorpion, archer (specifically his bow), goat, water-drawer (specifically his water bucket), and fish. The cycle begins in Nissan and ends in Adar; this is why many sources discuss Adar Beis as not being subject to the influence of the stars (see the discussion of the war with Amalek in the same book).
I'm sure you see where I'm going with this. The sheep was the god of the Egyptians. When Egypt, represented by Aries, fell, the Jews turned to the next Zodiac, the bull.
Many other explanations are presented there, though off the top of my head I believe they've all been covered by the other answers.