The name Isabeau does exist, although it's very rare nowadays. I'm sorry, but it's not the male form of Isabelle. Isabeau is the Old French for Isabelle. It has nothing to do with Belle. In Old French, some feminine names ended in "male" ways: Isabeau, Mahaut, Brunehaut, Marion, Berthe, Alix, Margot / Margaux... because Old French was a more Germanic, less Roman language.