Battleship (Man O'War/Quarantine) won the Grand National in 1938 and was the last entire horse to win the National. He retired to stud and sired 58 foals. He stood only 15.1 hh and is the smallest horse ever to win. He is also the only horse to win the Grand National and the American National.
I've got an ancient book with a photo of him and he was tiny - described as "no bigger than a modern polo pony"
The fences were really horrible in those days, he must have been a very brave horse. Northern Starlight, who won the John Hughes Chase in 2000, was only 15.1 hh.