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Missingno’s appearance in Pokemon Red and Blue can be altered based on the name you choose for your character

Ghost:  y is the 3rd, 5th or 7th character of your name

Aerodactyl skeleton: x is the 3rd, 5th or 7th character

Kabutops skeleton: w is the 3rd, 5th or 7th character

Default Missingno: none of the above conditions are met