Rockruff has 4 different evos. becoming Rock/Fire, Rock/Water, Rock/Grass, or pure Rock. Type: Null was a failed expirement on Rockruff, with Aether trying to make Rockruff evolve into a mix between all the evolutions. Sadly, this ended up killing a Rockruff, so they tried reviving it using a mask. This is how Type: Null came into existence. Gladion saw the monster that used to be his Rockruff, and left Aether.