Fatty Lumpkin

Fatty Lumpkin was a pony kept by Tom Bombadil. Fatty Lumpkin (sometimes called just "Lumpkin") was rarely ridden by Tom, and spent much of his time roaming free on the Barrow-downs. After their encounter with a Barrow-wight, Tom rode along with Frodo and his friends for a while on Fatty Lumpkin. The Hobbits' ponies had developed a strong friendship with Lumpkin during their time at Tom's house, and it was this that led them back to Tom Bombadil after they escaped from Bree.

Etymology

Tolkien notes that -kin is in this name used a diminutive suffix.