From Glitch City Laboratories
The Roaming Pokémon Roar glitch, sometimes shortened as the Roaming Pokémon glitch but not to be confused with the Roaming Pokémon individual values issue is a relatively minor glitch present within the Generation III Pokémon games, however through normal gameplay it only exists within Pokémon FireRed and LeafGreen because in those games, Raikou and Entei are the only Pokémon naturally capable of learning Roar by levelling up.
Usually when one of the Legendary Beasts flees, it should still exist within the game but would have fleed to a different route. If one of the Legendary Beasts uses Roar however, the game will not set this byte, suggesting that the player has run away or that he or she has defeated the Legendery Beast instead.
- Roaming Pokémon individual values issue