From Bulbapedia, the community-driven Pokémon encyclopedia.
Mount Hideaway (Japanese: ミヤザキ山 Mount Miyazaki) is an anime-only mountain located near Pallet Town. It is known for its population of unusually large Onix. Something in the soil causes the Onix to grow to gigantic proportions.
It is also a training area for Bruno. While training for the Indigo Plateau Conference, Ash and Brock decided to locate him and seek his advice on training in To Master the Onixpected!.
Pokémon seen at Mt. Hideaway