Some of the most imaginative coats of arms were invented for illegitimate children. They couldn't bear the arms of their father with a small difference like legitimate sons because this would suggest that they were legitimate, so new coats of arms were often created using devices from their father's arms.

Arms of Roger Clarendon
Roger Clarendon was the illegitimate son of Edward, the Black Prince - the eldest son of King Edward III. His arms derive not from the Black Prince's arms, but from his badge of an ostrich feather. An interesting example of the kind of differences between the arms of legitimate branches of a family and illegitimate branches are provided by the Beauforts. The Beauforts were the children of John of Gaunt, Duke of Lancaster (whose arms are above), by Katherine Swynford. Although John of Gaunt later married Katherine Swynford, their children were borne before this event and so were technically illegitimate. Later, they were legally "legitimised" and their arms were changed.



Illigitimate son of the “Black Prince”