Raikou did not originate from fanon, and the information on this page may not be entirely accurate to their canon. You may read more about this subject in the context of their canon on the following wiki(s):
Raikou is a Legendary Electric-type Pokémon from Johto. It is one of the legendary beasts, along with Entei and Suicune. Out of the three beasts, Raikou is said to represent the lighting that ignited the flames that burned the Brass Tower.
Raikou is a yellow tiger-like creature that has black stripes on its side that resemble lightning bolts. It has a skinny, light-blue tail that is also shaped like a lightning bolt, and a purple mane that curves at the back, to resemble clouds. Raikou has red eyes and long fangs.
Raikou is the only member of its trio not to be weak to another member of its trio. Entei is weak against Suicune and Suicune is weak against Raikou, but Raikou is not weak to either of them.
Due to a piece of artwork drawn by Pokémon designer Muneo Saitō, it is thought that both Suicune and Raikou were originally to be one "legendary beast". At this point in development, it is also possible that Entei did not yet exist.