In Pokémon Black and White, when players bring one of the Fateful Encounter Legendary Beasts to the Lostlorn Forest, an in-game event activates.
In the in-game event, you'll be approached by a camper, who would then growl and start a Pokémon battle.
The Pokémon battle starts with a Raikou/Entei/Suicune, but upon one hit it would be revealed that it was a Zoroark disguised using its Illusion ability.
As it turns out, the Raikou/Entei/Suicune at the start of the battle actually contains fully populated Pokémon data, that was obtained via a RAM dump.
These 'legendaries' are normally shiny locked.
Credits: @Poke J