Generation IV hybrids are glitch Pokémon that share the sprites of normal Pokémon. Before the battle starts,the Pokémon's sprite jumps around the screen in a line going left to right and they hover above the ground. They crash the game when their undefined Pokédex entry is viewed (some emulators can view the entry, but all cartridge versions will crash at this point), when they attempt to use their currently unknown glitch move, and when the player attempts to view their stats screen. All hybrids have a common moveset. They do not exist in Generation V because improved error handling capabilities withdraw the pointer data for species identifier modulo 651. This is also a similar case with Pokémon Battle Revolution, where any Pokémon with species identifier greater than 495 (or 0) is taken as (no Pokémon).
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Trivia
Any given Generation IV hybrid's hexadecimal identifier is the same as the corresponding Pokémon's, but with an 8 at the beginning instead of a 0 (e.g. Bulbasaur is hex 0001, its hybrid is hex 8001)
Generation IV hybrids can be caught in a cartridge version without the game crashing by using a Fossil modifier code.
Generation IV hybrids grow to the levels 1, 4, and 9 twice.
Generation IV hybrids skip levels 22, 30, 35, 44, 51, 56, and 62.
The Generation IV hybrids' unknown starting move is similar to the -- move, as it initially has no available PP.
At level 1, Generation IV hybrids attempt to learn an alternating sequence of Double-Edge and seemingly random move.
At level 42, Generation IV hybrids attempt to learn Signal Beam seven times.
At level 64, Generation IV hybrids attempt to learn the glitch move "-" ten times.
This glitch Pokémon article is part of Project GlitchDex, a Bulbapedia project that aims to write comprehensive articles on glitches in the Pokémon games.