John Gill Commentary Daniel 8:20

"The ram which thou sawest, that had the two horns, they are the kings of Media and Persia." — Daniel 8:20 (ASV)

The ram which you saw having two horns. Here begins the particular explanation of the above vision, and of the first thing which the prophet saw in it, a ram with two horns: which two horns, he says, are the kings of Media and Persia.

Darius the first king was a Mede, and Cyrus, that succeeded him, or rather reigned with him, was a Persian: or rather the ram with two horns signifies the two kingdoms of the Medes and Persians united in one monarchy, of which the ram was an emblem; (See Gill on Daniel 8:3).

For Darius and Cyrus were dead many years before the time of Alexander; and therefore could not personally be the two horns of the ram broken by him; nor is it to be understood of the kings of two different families, as the one of Cyrus, and the other of Darius Hystaspes, in whose successors the Persian monarchy continued till destroyed by Alexander, as Theodoret.