The Rainbow of Light is powerful magical artifact which Megan and the ponies first received it from the Moochick during the My Little Pony Special. It appears to be a simple red, heart-shaped locket, but when it is opened a rainbow streams out of it.
The Rainbow of Light defeated Tirek in the first special and later thwarted Catrina's first attempts to kidnap the ponies in Escape from Catrina, the second special. It also appeared in My Little Pony: The Movie and in the syndicated cartoon My Little Pony 'n Friends.
The concept of the Rainbow of Light was presumably invented by George Arthur Bloom, who wrote the scripts for both specials and the movie.
The Rainbow of Light is itself a precursor to the Elements of Harmony from My Little Pony: Friendship is Magic.