Entei, like all other legendary Pokémon, are not easily caught. If you are lucky, eventually you will still be able to catch him.
Steps

Step 1. First, you need to choose Bulbosaurus as your starting Pokémon
You won't be able to catch Entey using the codes. You will also have to defeat the elite four and get the national Pokédex.

Step 2. Buy 20 repels to scare away Pokémon

Step 3. You need a level 49 Pokemon, such a Pokemon can be found in the Kerulian Cave

Step 4. Make a Level 49 Pokemon the leader of your team
Use Repeel.

Step 5. Remember that Entey is constantly changing its location
He walks on different roads, caves and cities. If you fly, it will move even faster.
If you are lucky, Entey may be on the road you are heading for. You will need a Level 49 Pokémon and a Pokémon Scare item that will scare away all Pokémon below Level 50
Advice
- Entey cannot be found in the cave.
- If Entey loses consciousness, turn off the game without saving, otherwise you will never see him.
- The Sleep movement will help you.
- If you didn't choose Bulbosaurus, Squirtle will attract Pokémon Raiku, and Charmander will attract Pokémon Suikun.
- If you have a master ball, use it to prevent Entei from escaping.
- If you get tired of searching, pause and then try again.
- Use movements on the Pokémon that will leave him with one health point.