French here!
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.