Rockruff and starters share a secret. Let's list possibilities. We're clever enough together to work this out.

I'll start with the most obvious.

Rockruff and Starters both have split evos.

Evidence for:
Leak says Rockruff has split evolution. One is werewolf.
Every Pokemon can use Z moves.
Pokemon in past have evolved using day night feature. None revealed yet for games called Sun and Moon.
Werewolf associated with transforming under the full moon.
Pokemon with Standard and Alolan forms exist.

Evidence against:
Never done before.
Previous images show Pikipek as Pokédex entry 10.