You have to go to ecruteak city and meet eusine. Then entei won't be too far behind. You have to battle entei once you see him. You have no choice. Its not easy to beat him. I had to resort to my sixth Pokemon to defeat him!
after you catch a glimpse of entei, raikou, and suicune at the burned tower in ecruteak city, you can track entei and raikou using your map.
You can only catch it when steven talks to you in vermilion city when you try to get the doll for the copycat. You catch it the way you caught raikou and entei
you can find it in the grasses of johto after releasing it from the burned tower
Entei is a roaming legendary, to find it you have to go to Burnt Tower, and go to the basement. Once the legendary dogs have run away, you will be able to track Entei on your Pokegear.
In Pokémon HeartGold, the Legendary Pokémon that you can catch in it are Articuno, Zapdos, Moltres, Mewtwo, Raikou, Entei, Suicune, Lugia, Ho-Oh, Latias and Kyogre. You can also catch Latios if you have the Enigma Stone which is only available via a Nintendo Mystery Gift event and you can also catch Rayquaza if you have the Kyogre from "HeartGold" and the Groudon from "SoulSilver."
you cant catch entei on Pokemon pearl. entei is a johto Pokemon and Pokemon pearl is a sinnoh game. the only way to getentei is to trade it over from another game games compatible are Pokemon diamond, Pokemon platinum, Pokemon soulsilver and Pokemon heartgold. the last two i am sure you can get entei. (i have seen it in the wild)
you have to fight your way to the bottom of the tower. there, you will find suicune, raiku, and entei. then, when they see you, they will run away and you have to catch them(either catch or defeat, really).
after that, someone at the battle frontier will tell you about latias and latios. then, latias will start roaming kanto.
Ho-Oh Lugia Suicune Articuno Zapdos Moltres Mewtwo Kyogre Raikou Entei Latias
Entei is a powerful Fire-type legendary; however, it mainly flees in Pokemon FireRed, LeafGreen, Silver, Gold, Crystal, Heartgold, and SoulSilver. It wouldn't hit your Gastly, unless Mean Look is used. You can catch Entei with any Pokemon, because it just requires at least a Poke Ball to attempt to catch it. Be careful, though, if Entei doesn't run or get caught, it can easily defeat Gastly.
No you cannot catch Entei in Pokemon Emerald.
No, you can catch Entei without catching Moltres.