At the beginning, the game opens up with a menu (single player, multi player, etc.). Go to "Options", choose "Cheats." Enter the cheat, and press enter.
Cheat Code - Granted Wish:
BIGHEAD - Big Head Mode
MR HAPPY - No Dressage Failures
When entering the Main Menu of Equestriad 2001, click on "Options" then on "Cheats." Press "Enter Cheat." Type in Phar Lap. You'll get unlimited energy on the cross-country courses.