From Bulbapedia, the community-driven Pokémon encyclopedia.
Kirikiri Mountain (Japanese: キリキリ山 Mt. Kirikiri) is an anime-only location in Hoenn. It is located somewhere between Littleroot Town and Fortree City. The mountain is well known for their thick fog, limiting the vision around them.
The wall from the Baltoy civilization
On the mountain there is a mysterious cave that was the home of an ancient Baltoy civilization. The cave is hidden in an alcove in one of the forest's trees. Entering the cave, there are drawings on the wall from the Baltoy civilization, as well as an ancient structure that, through a Baltoy's Rapid Spin, allows a person and Pokémon to travel to another period of time.
Ash and his friends visited Kirikiri Mountain in Me, Myself and Time, trying to take a shortcut to Fortree City. They soon realized, however, that it would have been easier to go around the mountain, due to the dense fog. Ash got separated from the group and met Calista, and together they found the cave and the time machine.
Pokémon seen at Kirikiri Mountain