If you are playing the original Gold game:
You caught either a Missingno. (I'm pretty sure you didn't) or you caught the Red Gyarados. It's sad, but the Red Gyarados in Gold has a horrible glitch in it that has a chance of doing what you just described.
If you are playing Heart Gold:
I have no idea what happened. The Gyarados glitch does not occur in Heart Gold.
Well technically, you can't KILL a Pokemon. You can just make it faint. When you make a legendary Pokemon faint, it will be gone until you start a new game.
You cant he is gone from the game.
Yes, you can save the game before you catch it. but if you already saved it and the legendary is gone, then you can't catch it. if it is a Pokemon ranger game then you can catch it again
you beat the game and it's gone sure to bring Pokemon in the sixties as trainers are super strong
its gone it was destroyed by the volcano
it stays there in its pokeball forever like it did in the original Gold and Silver games. the pokeball is gone. the game says"no Pokemon left... you wonder who the professor gave it to"
You have to start a new game and overwrite the existing file when it comes to saving your new game since there's only 1 save file for the games so if you overwrite the existing file then it'll be gone for good.
Well technically, you can't KILL a Pokemon. You can just make it faint. When you make a legendary Pokemon faint, it will be gone until you start a new game.
It means production on the base game is complete aka gone gold.
Yes, when you migrate a Pokémon from a Gameboy Advanced game into a DS game, it'll be gone from the Gameboy Advanced game forever.
As you are unable to have multiple saves on Pokemon Diamond, that means that it does delete your old save. no you have to save first after pressing new game see if you press new game nothing happens to your old file it will go away if you press save while on the new game it will ask are you sure you want to get rid of the older save file if you say yes its gone forever and now you started fresh.
Sorry, Bud, but as far as I know, it's impossible. Once you delete the file, it's gone.
you can't. once it's deleted its gone. sorry you can't. once it's deleted its gone. sorry
You cant he is gone from the game.
No, once you migrate they are gone, and cannot be traded back, so beware if you really need that Pokemon on your D/P game.
I have gotten pokerus in sapphire as well but to cure it this may sound weird but it worked for me, if you train the Pokemon affected by pokerus then somehow your game must freeze up and when you reopen your file pokefus is gone! This worked for me and may not work for you.
Go on the title screen, then press Up, Select, and B at the same time. You'll be taken to the confirmation screen and tap Yes that you are sure you want to erase your current file. You must wait a few seconds, then it'll bring you back to the intro with your old file gone.