From Glitch City Laboratories
An example of a Pidgey affected by the Pokédex Species Glitch, its Species name should actually be the "Tiny Bird" Pokémon.
The Pokédex Species Glitch is a miscellaneous glitch which only exists in early versions of Pokémon Firered and Leafgreen. In the previous Generation III Pokémon games; the Species names for Pokémon were only one word. For example, in Pokémon Ruby and Sapphire the Species name for Pidgey was the "Tinybird" Pokémon, instead of the "Tiny Bird" Pokémon. This was possibly because of an error in the game engine because the game would ignore any words after spaces in the Species dialogue, so if Pidgey was named the "Tiny Bird" Pokémon, it would appear on the Pokédex as the "Tiny" Pokémon.
This is the case for the earlier builds of Pokémon Firered and Leafgreen, since Nintendo decided to give Pidgey its original species name as in Pokémon Red and Blue, a Pokémon's name will be "cut off" to the first word, since Pokémon Firered and Leafgreen almost have the same engine of Pokémon Ruby and Sapphire. If this glitch occurs in a player's Pokémon game he or she can be certain that he or she has version 1.0.
-  - An article with technical information about the Pokédex Species Glitch.