Nothing is known of the life of Guilluame, other than what he says or implies about himself in his writings. He calls himself a cleric (a religious office) of Normandy (northern France). He may have lived for a time in England, but he wrote in the Norman dialect of French, not in the Anglo-Norman dialect of England. His Bestiaire, or Bestiaire divin was written around 1210 or 1211; this date is derived from a comment in the text that it was written two years after England was put under interdict, which happened on March 23, 1208 by order of Pope Innocent III.
The Bestiaire is a verse bestiary of 3426 lines, making it the longest of the French bestiaries. It was very popular, with at least 23 manuscript copies, dating from the thirteenth to the fifteenth centuries, still existing; the Bibliothèque Nationale de France has 11 copies. Most of these are illustrated. Guillaume says that his reason for writing the poem is so that readers could benefit from the moral lessons in the animal stories. The text appears to be based on the early version of the Latin bestiary designated "B-Is", a combination of the "B" version of the Physiologus and material from Isidore of Seville's Etymologiae. Guillaume's moralizations closely follow the examples in this version of the bestiary, though he added to some of them. The manuscript Guillame used was possibly British Library, Royal 2 C. xii of the early thirteenth century, which is similar in chapter order and other details.
Other French or Anglo-Norman bestiaries are those by Philippe de Thaon, Gervaise, and Pierre de Beauvais.