there is not 1 certain route and you have to of chose squirtle to get entei
To get Entei on FireRed, you must trade from Colosseum on the Gamecube/Wii.
Entei can only be found in firered and leafgreen if you picked bulbusaur as your starter Pokemon at the beginning of the game and have the national dex and have delivered the ruby and sapphire to celio, then you can find it anywhere around kanto personally it sticks around route 1 you will find it easily there then throw your masterball at it. You can also get an entei from Pokemon colosseum after purifying it.
Go to the Pokemon center in 1 st ISland (after winning the league). Talk to the guy with the machine. Then go to MT. ember and find Ruby stone. Talk to that guy again. Then can you go to 4, 5, and 6 Island. And on one of that island can you find Entei
firered/leafgreen
If you choose balbasaur at the begin,you will meet entei after beating elite four(like meet latios in ruby)Go to four island to give birth to venusaur with ditto.After that, you will receive an egg.After the egg hatch,you bring the it to route 6(easy to find entei)walk in the grass for a while(about 20 minute)you will find entei.
you can find him around Kanto after beating the elite4 if you started with bulbasaur
it roams but u can only find entei in fire red.
If you chose bulbisor as your starter once you get the national dex entei is one of the three run away Pokemon
The earliest route you can get it at is Route 4.
Entei randomly appears in the world map after you have beaten the Elite Four, and if you chose Bulbasaur as your starter Pokemon -creepyjean- ^^ hehe
route 23
I found Entei at route 38 but it will take a while to find Entei because legendary Pokemon are very very very hard to find.