PK2: Unobtainable Trainer House Cal's Pokemon .

In Pokemon Gold, Silver and Crystal (and later in Pokemon HeartGold and SoulSilver), there is a location called the Trainer House.
The Trainer House allows you to battle the trainer in it once a day.
The trainer will normally be the trainer that you last used Mystery Gift with,
however if you never used that feature before, the trainer you get to battle is Cal.

Cal's Pokemon are the final stages of the Johto Starters.
It was discovered in the games programming (gen 2) that there are 2 unused teams,
which features the 1st and 2nd stages of the Johto Starters, but it is not known how to trigger these battle legitimately.

This page hold all 3 of the sets mentioned before, from Gen 2.