There were fifteen types introduced in glitches. Pokémon Red and Green had a tumultuous development, to the point where it's almost a miracle that they were released at all.
The first generation of Pokémon games went through a number of changes throughout production, and the final product resulted in some of the most unpolished games ever released on a Nintendo system. Pokémon Red and Blue were filled with bugs, and the battle system was horribly unbalanced, thanks to the overwhelming power of Psychic-type Pokémon. The leaked beta of Pokémon Red and Green revealed that a lot of ideas were considered and dropped throughout production, so it's unsurprising that the first generation of Pokémon games launched with issues.
One idea that made it far into the development of Pokémon Red and Blue was an unused type. Players know it made it far into development, as it can still be accessed in-game. This was the cut Bird-type, and it still exists, buried deep within the files of the original Pokémon games. In order to see a mythical Bird-type Pokémon, the player will need to call on the aid of the infamous MissingNo glitch.
In Pokémon, Bird Is The Word
According to Glitch City Laboratories, the Bird-type is a scrapped type that can only be accessed by summoning a MissingNo or M Pokémon. The weakness charts are all blank, so it's unclear exactly how it would have functioned on the battlefield. There are no Bird-type moves in the game. The Bird-type still exists in the files of Pokémon Gold and Silver, likely due to those games being built on the foundation of Pokémon Red and Blue's programming, but it was removed from Pokémon Ruby and Sapphire, and it has never been seen since.
It's likely that Bird-type was replaced with Flying-type, as the latter can encom Pokémon that didn't resemble birds. Looking at Kanto alone, future Pokémon games, but it feels as it would be redundant, especially where there are so many other possibilities for new Pokémon types.
Source: Glitch City Laboratories
