Literally tagged “the Mantle Pokémon”, roggenrola was apparently originally found in a fissure from an earthquake. However, if we look at its name for a moment, and roggenrola is truly found in the mantle, this Pokémon originated at Least 25km deep in the ground (or 5km if it lived under the ocean) but more than likely deeper.

Carbink is a Fairy/Rock type Pokémon that is said to have formed at high temperatures and pressures underground and slept for thousands of years until it is dug up in mines. The way that I interpret this is Carbink is also formed in the mantle. I believe this because of Carbink’s tie with diancie who is literally able to form diamonds and Diancie being a weird rare evolution of Carbink. Diamonds are only able to form at pressures and temperatures achieved in the mantle, and it has also been mentioned that carbink and roggenrola often fight one another. In other words, these Pokémon are occupying the same space (the mantle).

Honorable mentions: 

Larvitar

it is mentioned that larvitar is hatched deep within the earth and takes a long time to get to the surface (and it has to eat the equivalent of a mountain?)

Geodude

Roggenrola is also mentioned to fight geodude, but geodude has no mention of what depth it is formed at.