Then he is gone and you will have to trade with someone else to get him.
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No,But You Can Get Moltres,Zapdos,And Articuno.
You can find articuno in the poketch marking app when he's all around
Articuno Zapdos Moltres
If you've saved before attempting to capture it, DO NOT save it after you kill it. Instead, try again. If you didn't save before killing it or saved after you killed it, you have to transfer from FireRed or LeafGreen, or trade with someone who does have it.
The catch rate for the three legendary birds is 3 in all of the games, including Platinum.