The Minimize and Substitute glitch is a natural glitch in Pokémon Red and Blue. This glitch was fixed in Pokémon Yellow.
Enemy Chansey in the Unknown Dungeon know Minimize.
If after an enemy Pokémon uses Minimize, the player opens and closes the fight menu then view the stats of any valid Pokémon in the party, their sprite will become the 'minimized' sprite and the opponent's sprite will become a broken version of the Pokémon they viewed.
If the player views the summary of certain glitch Pokémon the opponent's sprite won't change but their backsprite still will.
The glitch Pokémon h POKé (hex:C3) is an exception; it is possible to view its front-sprite with this glitch. LM4 (hex:C6) is another exception that lets the player view its front-sprite.
The Substitute glitch occurs when the player opens and close the fight menu then views the stats of any valid Pokémon in the party after the foe uses Substitute. It causes the user's sprite to become the Substitute back sprite.