First off, this video

It shows that Zoroark can speak human languages

Now, that could just be an interesting quirk, but take into account the Sinnoh Legends. Specifically this one:

There lived a Pokémon in a forest.
In the forest, the Pokémon shed its
hide to sleep as a human.
Awakened, the human dons the
Pokémon hide to roam villages.

Now, what Pokemon is known to change its "hide" on a regular basis to blend in as needed?

Zoroark

So I propose that either Humans came from Zoroark who eventually got locked into the Human illusion, or Zoroark are Humans who have embraced their Pokemon side. In both cases, they forgot their origin over generations, leaving only myths, legends, and the occasional Zoroark that likes to pull mindfucks