Early Pokemon Black Screenshot

Speed Pokeball record...

Prof. Oak drawing

Sprite edit using memory editor...

Pokemon Card

Glitch Pokemon

There are many glitch pokémon. The most well known of all of them is MISSINGNO.
MissingNo. stands for "missing number". It is an error handler for when the game tries to access pokémon data that doesn't exist. If you encounter it, the game saves automatically, causes the Hall of Fame to mess up, and it can cause graphical problems.