An unstable hybrid Pokémon is a Pokémon in either Generation I or Generation II that simultaneously has the properties of two different species of Pokémon. An unstable hybrid Pokémon is composed of a "recipient" Pokémon—the Pokémon it truly is and will revert to—and a "donor" Pokémon. This is due to there being two species addresses for the player's party Pokémon in Generation I and II. The first address manages data for the "donor", whereas the second address manages data for the "recipient".

In Generation I, unstable hybrid Pokémon can be created by the Pokémon merge glitch, Rhydon glitch and Time Capsule exploit. In Generation II, unstable hybrid Pokémon can be created by the Pokémon merge glitch, or in the Japanese versions of Gold and Silver by the Bug-Catching Contest glitch.

Properties

The properties of unstable hybrid Pokémon differ between slightly Generation I and Generation II.

In Red, Blue, and Yellow

In Generation I, an unstable hybrid Pokémon displays the sprite, mini-sprite and type of the donor Pokémon, and evolves in the same way as it. It has the original name, learnset, TM/HM compatibility and effective type combination of the donor Pokémon.

An unstable hybrid Pokémon has the color palette of the recipient.

In Gold, Silver, and Crystal

In Generation II, an unstable hybrid Pokémon displays the sprite outside of battle, the minisprite, and type of the donor Pokémon, and evolves in the same way as it. An unstable hybrid Pokémon also has the learnset of the donor, the TM/HM compatibility of the donor and a gender outside of battle calculated based on the donor.

An unstable hybrid Pokémon has the sprite in battle, gender in battle and effective type combination of the recipient.

Stabilizing

Stabilizing an unstable hybrid Pokémon is the act of causing both Pokémon species bytes to become the same. The Pokémon may retain changes that happened to it while unstable, such as the Pokémon knowing normally impossible moves. To stabilize an unstable hybrid, a Pokémon can be evolved (in the way the donor would), traded to Pokémon Stadium 2 (this makes both species bytes match the recipient), or deposited and withdrawn from Pokémon Day Care (this makes both species bytes match the recipient). In both Generation I and Generation II, if an Evolution stone is used the 'ABLE/NOT ABLE' message is shown is based on the recipient Pokémon, but a donor Pokémon that can be evolved by the relevant stone will still be able to evolve even if the 'NOT ABLE' message is shown. An unstable hybrid Pokémon cannot be traded to Pokémon Gold, Silver, or Crystal; once stabilized, the Pokémon can be traded to the Generation II games.

Glitches which create unstable hybrid Pokémon

Bug-Catching Contest glitch

Performing this glitch allows the player to receive mainly a full copy of the last Pokémon, but with the second Pokémon species as the donor and the last Pokémon species as the recipient.

Rhydon glitch

When a glitch Pokémon with an invalid Pokédex number (valid numbers are 1-151) has a Pokédex entry displayed after capture and is sent to Bill's PC, it will become a hybrid with Rhydon (it is the recipient, Rhydon is the donor).

Pokémon merge glitch

When 'M (FF), Q ◣ or ????? (FF) is used to merge two Pokémon together, the result is an unstable hybrid of the two species.

Time Capsule exploit

When a Pokémon is hidden by ????? (FF) to bypass the Time Capsule party check and traded to Red, Blue, or Yellow, it will be a hybrid of the equivalent Generation I Pokémon (as the recipient) and 'M (00)RB/3TrainerPoké $Y (as the donor).

Cloning glitch

When cloning Pokémon by exploiting the computer storage system in Generation II, there is a chance to get a bad clone. This Pokémon is an unstable hybrid of the Pokémon you cloned and ?????.

Using unstable hybrids to level past 100

If the donor and recipient Pokémon have different experience groups, the unstable hybrid can level past 100, or drop down levels if the donor Pokémon was level 100. If the donor Pokémon requires more experience points to reach level 100 than the recipient, the hybrid can be leveled up until it reaches the amount of experience the donor would require. If the donor Pokémon requires less experience than the recipient to reach level 100, then the hybrid will level down to the level it would be in the recipient's experience group if it had the amount of experience the donor requires at level 100. Stabilizing the hybrid will preserve the level.

This occurs because the amount of experience is capped to the donor's experience group's maximum amount of experience, but the levels are calculated using the recipient's experience group.

Using unstable hybrids to make Pokémon have unusually high stats

If the glitch Pokémon X ゥ- xゥ,, whose stats reach around 400 each by level 100, is captured and then fused with another Pokémon (with X ゥ- xゥ as the recipient and the other Pokémon as the donor), the hybrid Pokémon will have unusually high stats as a result.

Using unstable hybrids to train MissingNo.

Normally (at least in Pokémon Yellow), MissingNo. cannot be trained above level 1. However, if MissingNo. is fused with another Pokémon (with MissingNo. as the donor) the resulting hybrid Pokémon can be trained and leveled up in battle, even past level 100 using the method above.

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