Regigigas is a legendary normal Pokémon.
How to obtain Regigigas: Edit
Regigigas can be found wandering in the wild after you collect all 3 of the "Regi's". (Regice, Registeel, Regirock.)
To obtain these "Regis", you will have to go to the Frostveil Catacombs and complete all the quests that are required to obtain these "Regis". Once you are done collecting all of these "Regis" you will have to head to Route 14. Once you are there you will have to enter the ruins and find the door with the 3 colors of the Legendary Titans (Regis). You will have to have the 3 Pokémon inside your inventory before approaching the door. Once the door opens you will be greeted with a cutscene with Regigigas. He breaks through the wall and escapes (you can not go through the hole he creates). After the cutscene, a message appears saying that you can now encounter Regigigas in the wild.