>>29146276It's not a theory, it's stated that Type Null was made from other Pokemon by humans. What isn't stated is which Pokemon, because the parts don't look like any of the starters or their evos, then they think that rules them out, but it's never stated that it was parts of them put together.
But it's more then likely the Starters and Rockruff because it goes with the alchemy theory. Salt (Rowlet), Sulfur (Litten), Mercury (Popplio) and Lead (Rockruff). These the the elements in the alchemic transmutation. The three elements and the base metal to try to purify it and turn it into Gold or a Philosopher's stone.
And since we know that Silvally's ability is literally named after Arcesus and Silvally's name sake comes from Saviour, as in Literally being Jesus Christ, son of God. It's easy to see what the intention was with it's creation.