The player character encountering the Hooked Metapod.
The Hooked Metapod is the official mascot of Glitch City Laboratories.
This Pokémon originates from a glitch in the English versions of Pokémon Red, Blue, and Yellow in which address $D05F ($D05E in Yellow) is set to a value other than 0 and a battle with Metapod is initiated. When $D05F is greater than 0, it results in the opponent Pokémon being interpreted as hooked (as if caught with a rod).
Usually it occurs as a side effect of an unterminated name glitch item or applying a PP Up to - (move) at specific locations. These locations must have a 0x50 sub-tile after BGB coordinates x=10 y=0C (affecting both $D059 ($D058 in Yellow), the instant encounter address and $D05F ($D05E in Yellow), the 'is this Pokémon hooked?' address). This Pokémon is always Metapod due to coordinate x=0A y=0C being the hex:7C menu tile that never changes.
Notably in the Japanese versions, the Japanese unterminated name glitch item instant encounter glitch allows for encounters of hooked Pokémon other than Metapod. This is because the menu tile responsible for instant encounter address $D036 is a sub-tile at coordinates where its identifier may vary (it is one of the current map tiles).
Another two notable hooked Pokémon are the hooked Dragonite and hooked MissingNo..