What ever happened to the mythical Pokemon raiu?

There seems to be a misunderstanding. There is no Pokémon named "Raiu" in the official Pokémon universe. It's possible you're thinking of:

* Raichu: The evolved form of Pikachu, an electric-type Pokémon. It's a well-known and popular Pokémon.

* A fan-made creation: There could be fan-made artwork or stories featuring a Pokémon named Raiu.