Rembrandt painted a young man (his son Titus?) in a Franciscan habit in 1660 (Rijksmuseum, Amsterdam), and there is a third portrait study of a Franciscan, dated 1661 (Helsinki, Atheneum). In their own day these paintings were probably considered to be 'tronies' (literally, heads), which were not meant to depict identifiable individuals. The sitter is instead depicted in a particular role, or as a character study. Pictures of this kind were popular among 17th-century collectors and can often be found in contemporary inventories.
The National Gallery, Trafalgar Square, London WC2N 5DN