What game are you playing?
Where does this Pidgeot come from?
Did you catch it? Was it traded? Is it from another country?
Are you using an emulator or a real cartridge?
Those are informations you should be telling us in the question and thru the tags because they directly interfere with the player's ability to give and change a nickname.
The way things are now, I don't think someone will be able to help you.