Question

Say it was a Darmanitan. Will it dissapear, or will it just not be registered in the Poke'dex?

Comments

I don't mean migrating, I mean if a friend traded with me.

Answer 1

You can't, it's impossible, Pokemon can be migrated from 4th gen to 5th, but not traded.

Source: Multiple websites, including numerous Q's on the db, and I've actually tried it

Comments

No, I mean if my friend traded it to me, not migrating.

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You literally can't

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Oh, okay.

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B3N is right but go ahead and try it if you want to mess with your DS'

Answer 2

It's impossible, if you trade Pokemon from Pokemon DS games (before B/W) it says if you trade these Pokemon, there is no way to get them back. So it's impossible.