When you trade a Pokémon from a Gen 1 game (Red / Green / Blue / Yellow) to a Gen 2 game (Gold / Silver / Crystal) using the Time Capsule feature, they'll often appear holding an item in Gen 2.
Listings state that transferring a Pikachu from Yellow will result in it holding a Light Ball when transferred – a usually very hard to obtain item – and typically the Pikachu partner you start with is the only one you'll find in the game.
However, if I were to trade a different Pikachu over to Yellow (from another Gen 1 game) and then transfer that to GSC, would it be holding a Light Ball, or does it only work for the starter Pikachu? I'm a little loathe to transfer my original Pikachu as whilst I can safely trade it back, trading resets its friendship back to zero. I don't want to upset the little guy!
Best Answer
As listed in This description of the light ball in Blubapedia it is only the starter Pikachu which will grant you a light ball whilst trading to generation 2. Its under the section Acquisition for GSC.
That makes sense since those special starter Pokemon are considered special Pokemon in game, much like the starters from the Let's Go Series. Every other Pikachu traded is flagged as a common Pokemon, since it has a catch location and catch level registered.
It is similar to shiny Pokemon, which will be determined as such during the catch, even if you can't see it on the standard generation 1 game.