Question: how did fiyero become the scarecrow?
Answer: In the musical, Wicked, Fiyero turns into the Scarecrow after Elphaba chants a spell to save his life during the song, No Good Deed. He can be seen for the first time during the Finale song when he opens a trap door to release Elphaba. The pair leave Oz to start a new life, as both of them are presumed dead.