The series revolves around a boy named Keep who has no memory of his past, and happens to be in possession of an artifact called the Spin-Shell - which allows him to control legendary beasts called... BEAST. Joining him in his quest are Jay, Alice, Mac and Toto, who each obtain their own Spin-Shells as the series goes on. Also, they can merge with the... BEAST... to form powerful hybrid warriors.
Western Animation / Beast Keeper
The best way to describe Beast Keeper would probably be something along the lines of Digimon Tamers meets Pokémon with yo-yos and sub-par animation quality.