The nature concerning what YOPs are is unknown, but they appear during the switch glitch (Generation III) as a clone of the opposing Trainer's Pokémon.
Attempting to battle with them may lead to the player to the switch screen or the "(OPPOSING TRAINER) is about to use (POKéMON)." message.
Following this message, the player may send out another copy of a YOP which overlays the older YOP's sprite or one of their own Pokémon. [clarification needed]
If the player sends out one of their own Pokémon, its sprite may again overlay the YOP but the palette of the YOP behind it may change to match the new Pokémon.
Video of the switch glitch in Ruby, FireRed and Emerald:
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