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Well the GTS has been around a lot longer than Black and White, so moving this to Pokemon Gaming Central, which doesn't cover the first Pokemon you got on it. ^___^
Mine was a level 48 Emboar for mah Zekrom :D. Reset game after that anyway, so wanted to do GTS once. Never really use it seeing as its annoying to look for Pokes in my opinion on it >_<. Plus I dont really need any other Pokes than the ones I have, so I dont use it *shrugs*
Well...in the 4th Generation it was a Glameow...2 years after I got Diamond :P
In Pokémon Black it was either Snivy or Tepig, which then became Serperior and Emboar, before being put back up for trade to get Zekrom and Braviary, respectively.