Pokemon Stadium (Japan)

Note: This is the first Pokemon Stadium game from Japan, not to be confused with the North American Pokemon Stadium.

More damage:
Execute a fighting move, then hold A when the description of the move is displayed.

Surfing Pikachu:
Successfully complete the Level 1-30 or the Level 50-55 Masterball Cup in stadium mode without using any Rental Pokemon and include Pikachu in your team in all battles. Your Pikachu will learn Surf after one of those two cups have been completed.

Amnesia Psyduck:
Enter all 150 Pokemon in the Hall Of Fame (Winner's Mansion). A message will appear. You will get a Psyduck that knows Amnesia when you go to Professor Oak's Lab.
Information in this section was contributed by Mewlingirl.

Expert mode in mini-game:
Enter the Minigame, and select "Competition". Select one player and win five games in a row. A message stating expert mode is available will appear.
Information in this section was contributed by Mewlingirl.