Yes by migrating it from firered or leafgreen.
you cannot
The only way to get Celebi on Pokemon Pearl without AR cheats is via event distributions. However these events are no longer being held or supported for Pokemon Diamond, Pearl and Platinum.
migrate
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)
There are no cheats without action replay for pearl but there are glitches but if some of them require for you to do the Pokemon trading thing do NOT do them because the Pokemon company can ban you from trading.Just search Pokemon pearl glitches I know there's tons of them.
cant
in a fossil
You can't without trading, migrating or, cheats
Trading, Migrating, Catching the legit legendary Pokemon in pearl, etc.
You can't without using cheats.
You Have to trade from Pokemon Diamond, Pearl, or Platinum.
If you have log cabin ranch then you can receive Mew from Hayley(only pearl/diamond)