Regigigas is a legendary normal type Pokémon.
How to Obtain Regigigas: Edit
Regigigas can ONLY be found in the wild after obtaining the other Regis. (Regirock, Registeel & Regice)
To catch these Regis, you will have to go Frostveil Catacombs There are three quests to complete in the Catacombs, and each must be completed before you can open the three chambers scattered around the map. These chambers are Calcite Chamber which contains Regirock, the Martensite Chamber containing Registeel and the Dendrite Chamber containing Regice.
After successfully capturing the three Regis, put them in your party and then go to Route 14, and then proceed to the Titans' Throng only whilst having the three Regis will the door to Titans' Throng open. After opening the door, you will be put into a cutscene to which you can see Regigigas escape through a hole he breaks in the wall; it will now say you can encounter Regigigas in the wild! NOTE: You cannot go through the hole he creates.