My idea for an Ultra Beast

Hydranger (UB: Overgrowth)

Type: Grass/Dark

Category: Ultra Beast / Overgrowth Pokémon

Ability: Overgrowth Command — When Hydranger's HP is below 50%, the alpha head boosts the attack and speed of all heads.

Etymology: Hydra or hydrangea + danger

Description:

Hydranger is feared as UB: Overgrowth, an Ultra Beast that invades ecosystems to dominate plant life and shadowy realms. Its alpha head's commands are absolute, and it can coordinate its many jaws to overwhelm foes with relentless biting attacks. Legends say that when the alpha head blooms with leaves, it signals the rise of an unstoppable force in the wild.