The legendary beasts is the collective term used to refer to the trio of Entei, Raikou, and Suicune.
A direct counterpart to the Generation I trio, the beasts find their master in Ho-Oh, who revived them when Ecruteak City's Brass Tower burned. They are said to embody the three events that happened to the tower: the lightning that struck the tower, the fire that burned in the tower, and the winds and rain that put it out. It is unknown whether these three were already a Suicune, Raikou, and Entei before they were revived, or whether Ho-Oh actually reincarnated them from three non-legendary Pokémon.