Nihilego, known as ウツロイド in Japanese, is an Ultra Beast appearing in the "Pocket Monsters" series. It is a Rock/Poison-type Pokémon that resembles a jellyfish and is known for its translucent, glassy appearance. Nihilego plays a significant role as one of the mysterious Ultra Beasts, creatures from another dimension called Ultra Space. It has the ability to latch onto and control other beings, creating a unique and perilous dynamic in the storyline. Its presence in the series often leads to key plot developments and challenges for the main characters.