Unstable hybrid Pokémon are glitched Pokémon in Generation I and Generation II which share the properties of two Pokémon. There are various glitches which allow the player to obtain an unstable hybrid Pokémon, such as the Pokémon merge glitch.
After 'fusing' a Pokémon, it will have two species bytes which do not match. Its first species byte is considered to be the "donor" byte and its second species byte is considered to be the "recipient" byte (see the mechanics section below).
An unstable hybrid Pokémon is not allowed to be traded into Pokémon Gold, Silver and Crystal, as unstable hybrid Pokémon are considered to be "Abnormal" Pokémon.
Glitches which allow for unstable hybrid Pokémon
In a Pokémon's data structure there are two species bytes. For example, the memory addresses D164 and D16B in Pokémon Red and Blue (D163 and D16A in Yellow) both control Pokémon 1's species.
The first byte (e.g. D164), is referred to as the "donor" byte and the second byte (e.g. D16B) is referred to as the "recipient" byte. When the player performs the Pokémon merge glitch it causes the game to combine one Pokémon with another. The original Pokémon is the recipient and the Pokémon above it (to be fused with the original Pokémon) is the donor.
The table below describes the properties of donor and recipient Pokémon in Pokémon Red, Blue, and Yellow and Pokémon Gold, Silver and Crystal.
||Sprite, menu-sprite, type displayed on the menu, evolutions, learnset,|
TM/HM compatibility, experience cap.
||Pokémon's palette, Pokémon that will be received when taken into|
the Day Care or Stadium 2, experience group, base stats.
||Sprite outside of battle, menu-sprite, evolutions, learnset,|
TM/HM compatibility, experience cap, calculated gender outside of battle.
||Sprite in battle, calculated gender inside of battle.
Leveling above 100
Normally finishing a battle after having a Pokémon that is stable and over level 100 will cause it to drop back to level 100. However, using an unstable hybrid Pokémon it is possible to finish a battle with a Pokémon and keep it over level 100.
In Pokémon Red, Blue, and Yellow, the 'donor' byte controls a Pokémon's experience cap and the 'recipient' byte controls a Pokémon's experience group. If the Pokémon is given a higher experience cap than the usual maximum experience for that Pokémon's experience group, it is possible to level it above 100. Note that if the Pokémon is below level 100 Rare Candies cannot be used to raise the Pokémon's level above 100.
An example of how an unstable hybrid Pokémon can grow to above level 100 is Chansey fused with a Mewtwo (a Mewtwo/Chansey hybrid). Since Mewtwo is the 'donor' byte, its experience cap will be Slow with a maximum of 1,250,000 experience. Chansey, (the 'recipient') is part of the Fast experience group in which normally the amount of experience for level 100 is 800,000. Since the Mewtwo/Chansey hybrid has a higher experience cap, it will be able to attain a maximum 1,250,000 experience calculated using Chansey's experience group; allowing for a maximum 'permanent' level of level 116.
Raising certain glitch Pokémon above level 1
Fusing a glitch Pokémon which normally cannot reach level 100 (with a high experience cap but an experience group which means it cannot level above 1) with a Pokémon that can normally reach level 100 will allow the player to raise Pokémon such as Yellow regular Missingno. above level 1 by altering its experience group. For example, Chansey (the recipient) can be hybridized with Yellow regular Missingno. (the donor) to become a Missingno./Chansey hybrid, which can then be leveled up to a maximum of level 254 (finishing the battle with this Pokémon at level 255 may cause the game to lock-up for unknown reasons).
Creating unusually strong Pokémon
Fusing a Pokémon with high base stats with a Pokémon with relatively low base stats (e.g. creating a Magikarp/Mewtwo hybrid; Magikarp is the donor, Mewtwo is the recipient) will allow the player to appear to battle with a Pokémon with the sprite of a relatively 'weak' Pokémon such as Magikarp. Note that while this may work for the Pokémon's summary screen in Gold/Silver/Crystal, it would not work within a battle; where the Pokémon in the Magikarp/Mewtwo example would look like a Mewtwo.
Creating new Shiny Pokémon
Fusing a Pokémon with DVs that dictate it as a Shiny with another Pokémon will allow the player to create a donor Pokémon/original Shiny hybrid.
The donor Pokémon may be evolved to stabilize it into a new Shiny Pokémon, for example the Red Gyarados may be hybridized with a Charmander to become a Shiny Charmander/Gyarados hybrid. The Charmander/Gyarados hybrid will then be able to evolve into a stable Shiny Charmeleon.