The decision to construct a "new"class of locomotives utilising the frames from 'Bulldog' 4-4-0s together with the boilers, cab fittings and smokeboxes from members of the 'Duke' class began in 1929 when 'Duke' 3265 Tre Pol and Pen was officially withdrawn and its boiler and fittings were used on the frames from 'Bulldog' 3365. The rebuilt engine survived in its new form until December 1949.
Bewteen 1936 and 1939 boilers and fittings from the remaining 39 'Dukes' were used on a number of 'Bulldogs' as they came into Swindon Works. The 'Dukedogs' as they became to be known were initially numbered from 3200 onwards but in 1946 were renumbered from 9000 onwards to make way for new 2251 0-6-0s.
9000 was originally named Earl of Mount Edgecumbe but as with the twelve other members of the class which carried 'Earl' names, the plates were removed in 1937 and the names used instead on new 'Castle' class 4-6-0s then entering service.