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Well I'm not much bothered either way, but we could give them something like:
Frighten Beasts (Ex): A monoceros can howl horribly as a free action, any animal within 120 feet that has 4 HD or fewer Hit Dice must make a DC 13 Will save or become frightened. This is a sonic, fear-based mind-affecting effect. The save DC is Charisma-based.
I'd rather the duration had a random element. Perhaps 4d6 rounds, like a dragon's Frightful Presence? Or cut it down to 3d6 rounds if you like an average duration around a minute.
As for the size, some combination of the SRD Unicorn and Rhinoceros would seem fitting.
Unicorn - "A typical adult unicorn grows to 8 feet in length, stands 5 feet high at the shoulder, and weighs 1,200 pounds. Females are slightly smaller and slimmer than males."
Rhinoceros - "6 to 14 feet long, 3 to 6 feet high at the shoulder, and weighs up to 6,000 pounds."
How about "A typical adult monoceros grows 7 to 12 feet in length and stands between 4 and 6 feet high at the shoulder. Most weigh about 2,000 pounds, but they can weigh up to 6,000 pounds. Females are slightly smaller and slimmer than males."