Here are the ways you can encounter Entei:
* As part of the legendary beasts' story: Entei, Raikou, and Suicune are the central focus of the game's main story. They appear at various points throughout the game and can be captured.
* Via encounters: After completing the main story and capturing all three legendary beasts, you can encounter them again in various locations around the Johto region. This requires using the "Rare Pokémon" option on the game's "Option" menu.
So, while Entei has a fixed appearance in the story, it can be encountered more than once through these later encounters.