No, Deoxys is only in Emerald, and you need to migrate it to can catch it. As for Celebi and Mew, you need a cheat code for them.
you get it by an event.
You need to have the Celebi event. it is over. sorry
No.
Mystery Gift.
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you will have to transfer one from Pokemon diamond, pearl,platinum,soulsilver,and heartgold
The Forest Guardian Legendary Pokémon is Celebi and you can only get it from an upcoming event for Pokémon Diamond, Pearl, Platinum, HeartGold and SoulSilver that has yet to be announced for countries outside of Japan.
I'm afraid you cannot get a Celebi in Pokemon HeartGold, unless you use a Action Replay. The only Celebi legal, I think, is the Nintendo Event that has passed. If you do have the Celebi in your GameBoy Pokemon Cartridge, then you can migrate it to Pal Park, then trade from Pearl/Diamond to HeartGold/SoulSilver.
You Can Not Catch Celabi In Pokemon Firered Unless you trade with pokemon heartgold or soulsilver.
You can get it in Pokemon Black and White if you send a special Celebi from Pokemon Heartgold, Soulsilver, Pokemon Platinum, etc.
You cannot CATCH Celebi on Pokemon HeartGold or SoulSilver, but there is an wifi event a Gamestop that gives you a Celebi in mystery gift. You can also use a Action Replay™, but I suggest you do the event.
The Celebi event for HeartGold and SoulSilver has already came and went. It started on February 21, 2011 but it ended on March 7, 2011. There is no chance of the Celebi event download for HeartGold and SoulSilver repeating so the only way you can get the event Celebi is by trading for it.