Altered Beast: Guardian of the Realms is an action game for the Game Boy Advance, and is a single player experience. It was developed by Sega and published by THQ.
It should be easy to find Altered Beast: Guardian of the Realms for the Game Boy Advance available for purchase. In good condition, the game is worth about $29.99. Altered Beast: Guardian of the Realms was released on 11/26/2002.
Press Start at the title screen. Hold L or R at the main menu to control the background's movement. Hold either button longer to move it faster.