Entei is a Legendary Pokémon that is exclusive to Pokémon Gold and Silver (and their remakes, HeartGold and SoulSilver). It is one of the three Legendary Beasts, along with Raikou and Suicune.
You can encounter and potentially capture these Legendary Beasts in Pokémon Gold, Silver, HeartGold, and SoulSilver by following specific steps and using certain items.
Unfortunately, they are not available in FireRed.