Although its name was Type: Null at first, the boy who evolved it into this form gave it the name by which it is now known.
– Pokemon Moon Pokédex
Apparently Gladion named Silvally, wonder how he told the Pokedex.
Strangely, Silvally only gains a bit more Speed upon evolving from Type:Null. It has 95 BST in every stat.
Its ability is basically Arceus’s (the name of the ability is even AKS, = “arceus”). I’m still a bit salty that they made a set of all new items for Silvally, instead of just, you know, recycling Arceus’s plates, since they do exactly the same thing.