Total items changing glitch Pokémon
This effect is caused by the glitch Pokémon having a Pokédex number between 153-160. This causes the game to interpret a bit of D31D (Red/Blue)/D31C (Yellow) (number of bag items) as the Pokémon's encounter flag.
There is only one total items changing glitch Pokémon in English Pokémon Red, Blue, and Yellow. This is 4( h 4 ? (ED), which may add +64 to the number of items in the bag upon encounter or capture. However, 4( h 4 ? (ED) has a volatile sprite that may freeze the game, and it also cannot be captured due to its experience group 0xFF, which causes the experience division by 0 glitch. However, unstable hybrids obtained with 0xED as the first species byte do not have the latter problem.
One known stable total items changing glitch Pokémon can be found in another language of Pokémon Red, Green, Blue, and Yellow. This is ヂィﾞィﾞゃいﾞ(hex:ED), which is possible to encounter without freezing the game.