Arceus is known as the 'Alpha God Pokemon'. It is known as the god of all Pokemon. It is not obtainable ingame.
Arceus can change its type with the plates that are obtainable in Lagoona Trenches. The only other Pokemon to do this as well is Silvally (Gen 7)
Arceus has 120 in all its stats, making it one of the strongest Pokemon ever. It is a mythical Pokemon.