I find it curious that neither Frank Miller's graphic novel 300, nor Herodotus's account of Thermopylae, The Histories, ever mentions a rhinoceros, yet this film feels the need to throw one in. The success of the Spartan stand was because Xerxes was trying to push his massive army through a narrow passageway where indefatigable warriors stood. There was no confrontation between Xerxes and the Spartans (as in the graphic novel), nor did Xerxes "frantically pull out wild cards" to dislodge them as this website mentions. He just ordered his hired soldiers forward from a safe spot in the middle, and only at the end of the day realized the scale of the slaughter.

Have we reached so low a point in cinema that the greatest ever act of wartime bravery has to be augmented with fiction?

but...
rhinos are neat.

besides, a more realistic portrayl of the events would probably be really boring. Imagine one character fighting dozens of contiguous swordfights and winning, until he doesn't. Then switch focus to another guy in a dress who does the same. Yawn.

Rhinos are the spice of life. Add a naked chick riding it, and you have gold.