Father: William Ferrers, Earl of Derby

Mother: Sibyl de Braose

Associated with John , King of England

Child 1?: Joan of England (possibly)

One source for the speculation that Agatha Ferrers was Joan's mother is Sir William Dugdale, in The Baronage of England (1675-6). He states his source to be Dr David Powel's History of Wales (1584). Powel's work is an enlarged edition of H Lhoyd's translation of The Historie of Cambria by the 12th century Saint Caradoc of Llancarfan.