Isabel f. Old Provençal Elisabel 'Elizabeth'.
The name of a 12th C queen of France, a 12th-13th C queen of Jerusalem, a 13th C French saint, a 13th C queen of Navarre, a 13th C queen consort of Aragon, a 13th C queen consort of France, a 13th C queen consort of England, a 13th C queen of Jerusalem, a 13th C Holy Roman empress, a 13th c queen of Armenia, two 13th C queens of Cyprus, a 13th C Hungarian saint, two 14th C queens consort of England, a 14th C queen consort of Germany, a 14th C queen consort of Portugal and saint, a 14th-15th C queen consort of Castile, Leon, and Aragon, a 14th-15th C queen consort of France, a 15th C queen consort of Portugal, a 15th C queen consort of Castile and Leon, a 16th C queen consort of Sweden, Denmark, and Norway, a 16th C queen consort of Hungary, and a 16th C Holy Roman empress and queen of Spain and Italy.
Though in origin a form of Elizabeth, and sometimes even by the 17th C still used as a pet name of it in England, it very quickly became considered a distinct name.
The English diminutives Ib, Ibbot, and Ibot are witnessed in 1379 in the relational bynames Ibsoñ (WRYPT1 p. 18), Ibbotsoñ (WRYPT1 p. 156), and Ibotsoñ (WRYPT1 p. 28). These pet forms can also be diminutives of Elizabeth.