* After defeating the Elite Four and becoming Champion: Entei will appear in the Burned Tower in Ecruteak City. This is the most common and reliable way to find it.
* After obtaining the National Dex: Entei will have a chance to appear in random encounters in Kanto (after transferring to Kanto). This is a rare occurrence, and you may need to spend some time searching to find it.
* After defeating the Elite Four and becoming Champion: There is a rare chance that Entei may appear in the Johto region at random. This is much less likely than finding it in the Burned Tower, but it's possible.
Important Note: You can only catch one Entei per save file.