It's possible you are thinking of Entei, which is a legendary Pokémon that can be caught in Pokémon Crystal. You can find Entei in the Burned Tower after defeating the Elite Four and becoming the Champion.
Here's how to find and catch Entei in Pokémon Crystal:
1. Defeat the Elite Four and become the Champion.
2. Visit the Burned Tower. This tower is located in the southwestern part of the Kanto region, near the Lake of Rage.
3. Entei will appear inside the tower. It will be wandering around, so you may need to explore a bit to find it.
4. Prepare to battle Entei. It is a powerful Fire-type Pokémon, so bring strong Water or Ground-type Pokémon.
5. Catch Entei using Poké Balls. This may take a few tries, as legendary Pokémon are notoriously difficult to capture. You can use items like Ultra Balls, Master Balls, or Dusk Balls to increase your chances of catching it.
Good luck catching Entei!