Eagle. This symbol is one of the most ancient. With the Greeks and Persians it was sacred to the Sun; with the Egyptians, under the name of Ah, to Horus, and the Kopts worshipped the eagle under the name of Ahom. It was regarded as the sacred emblem of Zeus by the Greeks, and as that of the highest god by the Druids. The symbol has passed down to our day, when following the example of the pagan Marius, who, in the second century B.C. used the double-headed eagle as the ensign of Rome, the Christian crowned heads of Europe made the double-headed sovereign of the air sacred to themselves and their scions. Jupiter was satisfied with a one-headed eagle and so was the Sun. The imperial houses of Russia, Poland, Austria, Germany, and the late Empire of the Napoleons, have adopted a two-headed eagle as their device.
Source: H.P.Blavatsky - The Theosophical Glossary