Entei is a Legendary Pokémon introduced in Pokémon Gold and Silver. It, along with Raikou and Suicune, are not available in the original Generation II remakes, Pokémon FireRed and LeafGreen.
You can, however, encounter Entei in Pokémon FireRed and LeafGreen with the help of Pokémon Trading Cards Online (PTCO). By linking your game to PTCO, you can receive a code for Entei.
Keep in mind:
* This method is no longer possible as PTCO has been discontinued.
* This method only applied to Pokémon FireRed and LeafGreen, not to Pokémon LeafGreen.
To find Entei in the Pokémon games, you need to play Pokémon Gold, Silver, or their remakes, HeartGold and SoulSilver.