From Bulbapedia, the community-driven Pokémon encyclopedia.
The Forbidden Forest (Japanese: 禁断の森 Forbidden Forest) is an anime-exclusive Grass-type Pokémon reserve that appeared in Grass Hysteria!. It is located somewhere between Littleroot Town and Fortree City.
No humans have been allowed in the Forbidden Forest after people repeatedly tried to catch all of the resident Pokémon. Consequently, the Pokémon grew increasingly hostile towards humans. As a solution, some people established a village, currently led by Old Man Obee, to watch over the area, and also permanently sealed off the forest with a huge wooden fence. A Venusaur is the "boss" of the Forest. The area's peculiar magnetic field, which affects electronics and navigation tools, ensures many travelers are unaware of the Forbidden Forest.
May was kidnapped by a Skarmory and was dropped into the forest. While exploring, May met a Bulbasaur trying to pick flowers. It warmed up to her and defended her from the other Grass Pokémon, who saw her as a threat. When May left the forest, Bulbasaur decided to go with her.
Pokémon seen in the Forbidden Forest