The glitch Egg is Egg data with the Pokémon specified to hatch from it as another Egg, and thus acts as a glitch Pokémon.
Egg data in Pokémon games usually have their own entry in the same chunk as other species' data. In Generation II, number 253 stored most of the data associated with the unhatched Egg. For any Pokémon in a Trainer's party, there are two values stored per Pokémon that indicate its species. The first value, found in the Pokémon data structure, decides how the stats grow and what sprite appears in battle. The second value is typically exactly the same as the actual species value while it is in the party that almost only decides what sprite appears in the party screen next to the Pokémon's name. However, when the Pokémon is still an unhatched Egg, this second value will be 253, signaling the game to treat it as an Egg. Unstable hybrid Pokémon are Pokémon with these two values set to different species.
It has a footprint with the number 253 embedded in it.
This glitch Pokémon is an Egg that will hatch into an Egg. Therefore, the Egg sprite will appear next to the glitch Egg like a normal Egg and the glitch Egg is forced to hatch into another glitch Egg every 30,720 steps.
This Pokémon both acts as an Egg and a Pokémon, depending on the circumstances. The player can deposit all Pokémon but glitch Eggs into the PC. Also, it is possible to give away the player's last Pokémon other than a glitch Egg by giving a Pokémon the Mail from Randy and delivering it to the man on Route 31.
The glitch Egg in battle
The glitch egg has no constant base stats or experience types. It instead takes this data from the Pokémon it is sent out against.
In Pokémon Crystal, it can battle but has no moves, so it will always use Struggle when told to attack. If it faints and the player whites out, it will not be healed. As long as the player has no conscious Pokémon in their party, all battles will be over right before the player sends out a Pokémon. The game behaves as if the player had won, but no experience is gained.