http://en.wikipedia.org/wiki/Thomas_Manny
Thomas Manny, also known as Thomas de Manny or Thomas of Manny was the only son and heir of Margaret, Duchess of Norfolk and Walter de Manny, 1st Baron Manny.
Biography
Born in 1357 in London, England,[1] he was the third son of his mother and only son of his father. He was heir to the Earldom of Marshal, County of Norfolk and Baronet of Manny. His two other half-brothers had died before being ten years of age; thus, he was his parents' only son.
Thomas died aged five.[2] He drowned in a well in Deptford, Kent[3] in January 1362.[1] He was succeeded as heir by his sister.