Aside from the standard fifteen Pokémon types in RBY, there is a sixteenth type, hidden in the programming. This type is named Bird.
The Bird type is quite enigmatic. Its attacks are physical. No attack is of the Bird type and no Pokémon is of the Bird type. There are no types super‐effective or not very effective against it, nor are any types weak or resistant to it. It’s just… there.
Interestingly enough, the filler Pokémon Missingno. was of the Bird type. However, Bird never comes into play outside of glitches.
This type was never removed from the source code, and exists in all games up to and including Pokémon Crystal.
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