EDIT: The Legendary Beasts are thought to be based on mythological creatures:

Raikou is a raiju (a.k.a. a tiger).

Suicune is a kirin (a.k.a. a wolf).

Entei is a shishi (a.k.a. a lion-dog fusion).

Please check the related links below to see an image explaining in more detail this theory, along some photos.

Raikou looks like it should be a member of the big cat family so personally I would place Raikou as a tiger cross with a lion and a lot of media magic.