Isabeau (IZ-ah-bow, EE-sah-bow) is a French variant of Isabel, derived from the Hebrew name Elizabeth (Elisheba/Elisheva), meaning "God's promise, my god is a vow/oath." There are more alternate forms of the name, including Ysabeau, Esabeau, Isabetta, Ishbel, Isabelle and Isobel. Beau in French is the masculine form of beautiful, or should I say handsome. One possible explanation for this masculine ending could be the way the Spanish translated Elizabeth, and the Occitan (or medieval Gascon language) version: Eisabèu.
While Isabelle was the name of several Spanish queens, Isabeau has its own namesakes. Isabeau of Bavaria, who may have been born "Elizabeth," was Queen Consort of France, wife of King Charles VI. She had quite an amount of power for a medieval queen as regent. There was a different Isabeau that was convicted of being a witch. Another well known Isabeau was the short-lived Isabeau of Brittany.
More recently than historical queens, Isabeau was an opera by Pietro Mascagni, a retelling of the Lady Godiva legend, and Lady Isabeau was a heroine found in the movie "Ladyhawke." She can also be seen in the form of heroic succubus Bo/Isabeau in the TV series "Lost Girl." A quick search on Youtube will reveal "La Belle Isabeau" by Berlioz, which I believe was based on a troubadour song from the 1100's AD. You'll even find a pretty model named Isabeau Hitijahubessy.
Never charting in the U.S., the baby name Isabeau remains rare, even in its native country. In 2011 there were just 13 girls given the name Isabeau, and there have never been more than 18 born in a year. It was, however, popular in the Middle Ages, and the name did make it to several countries.