It's a secret evolution. That means it's an evolution we haven't seen before. It's not a stone evolution, friendship level, level up, trade, moveset, or time of day evolution. The dex entry states:

>"Rockruff is a sociable Pokémon, but as it grows, its disposition gets wilder. If it begins to howl when the sun goes down, that is proof that it’s close to evolving. It’s said that it leaves its Trainer’s side to evolve and returns again when fully evolved."

It's literally in the dex entry. You don't just leave it on an island by itself to evolve like we've been speculating. I think we actually have to release Rockruff, track it down, and recapture it.

But this isn't the secret that Rockruff shares with the starters. Honestly, I think Rockruff has an influence on the starter's evolutions. I can't see each starter evolution having 4 different forms along with Rockruff's 4 forms.

Speculation Thread.