After reaching their third Evolution, Monsters stop earning Monster XP when their Needs are met. Instead, they earn Energy every time they are provided with a Need Item. The amount of Energy provided by a Need Item is the same as the amount of Monster XP it used to provide, and the same bonuses provided by Boosters and Habitats still apply. Once the Monster's Energy meter is full, they enter an Energized State.
While in an Energized State, a Monster will glow and sparkle. While in Battle, they are also guaranteed that every one of their attacks will count as a Critical Hit. Once the Battle is over, the Energized State is gone, the Monster returns to normal, and they can start earning Energy again. If a Monster is in an Energized State and doesn't enter a battle, the Energized State will not wear off.