I have a picture from serebii. After watching an episode of Houen. I didn't know that these were in the anime, but the Primal Pokemon were only in the special and I've been watching and looking around and doing my hard research.
In an older Pokemon season, this Pokemon sits at the bottom of the ocean and you can hear the man say the ocean got really hot and said it was the work of a Pokemon.
In this video, https://www.youtube.com/watch?v=T_HDWwqn79g I think it's the exact same type of Groudon sitting at the bottom of the ocean. Remember, I don't like claiming things exactly without proof as the first image was from an episode that was over 10 years ago. The youtube video was from a more recent game after Mega Evolution was introduced.