At the age of 18, in 1880, he started keeping a stallion, called ‘Bold Buck’, s. of ‘Cardigan Driver’ owned by a Unitarian minister at Maesymeillion, Llanybydder. When he was a servant at Pantmoch, Pontsian, he bought a 6-year old cob ‘Welsh Briton’, from David Charles Jones, Abercefel, Llandysul which became one of the taproot sires of Welsh cobs. Those were the days of the trotting cobs. ‘Welsh Briton's’ time for the mile was 2 minutes 18 seconds. Three of his progeny were champion trotters at Alexandra Park, London, during three successive years, 1892-3-4, and this was considered a considerable achievement. ‘Welsh Briton’ was 21 when he died. He is one of four cob stallions buried in the land of Cwmgwenyn, Llangeitho. A further 6 stallions lie buried in Blaen-waun. Of these 10 stallions 7 were over 20 years of age.

Thomas Rees was keeping Welsh cobs long before the establishment of the Welsh Stud Book in 1902. He has 2 stallions and 2 mares registered in that first volume. He kept 3 sons of ‘Welsh Briton’ as stallions — ‘King Briton’, ‘Briton Comet’ and ‘Britonian’. He kept Welsh cob stallions for 70 years. He led stallions himself and engaged others to do so. Even at 80 years temporarily re-entering the lists as he had given up ‘travelling’ years before, he took his black stallion ‘Blaenwaun True Briton’ round for one season, travelling three days a week.

His three sons followed in his footsteps. David Rees led his stallions round for 50 years and kept them for over 60 years, James Rees for a similar span and Harry Rees throughout his shorter life. Thomas Rees's stallions travelled as far as Glamorgan and Gwent, and on occasion into Gloucester, where Harry Hopton, the smith worked. They walked all the way. ‘King Briton’ was the first stallion which he travelled into England. The contribution of Thomas Rees and his sons to the Welsh cob is distinctive on three counts; (1) his stallions represented the oldest strain of Welsh cobs, (2) no one else gave such an aggregate of years to owning and leading cob stallions, (3) the family devoted itself exclusively to the Welsh cob even when a majority of breeders found it more profitable to keep other breeds. It has often been said that the Welsh cob would not have survived the lean years but for the contribution of Thomas Rees and his sons. He was made an honorary member of the Welsh Pony and Cob Society. John Roderick Rees, son of David Rees, inherited ‘Rhosfarch Frenin’ continuing the line of the family's cob stallions into its hundredth year.

He was a talented man despite his lack of early education, a keen reader, a genealogist and local historian and a fine story-teller. Though he spent the greater part of his life in the midst of Calvinistic Methodism as a member in Capel Gwynfil, he continued at heart to be an Independent, deriving great joy from Elfed's hymns. It was his Welsh cobs that kept his name alive, but he was also well-steeped in the culture of a vanishing world.