In the book The Lord of the Rings by J.R.R. Tolkien, Bill was a pony bought by Frodo Baggins and his companions in Bree, as they fled the Shire on their way to Rivendell. This was necessary as their own ponies had been stolen from the stables of the Prancing Pony inn, where they spent the night. Bill was purchased from Bill Ferny, who was in league with the spies who stole the other ponies. Ferny was a cruel man who tormented Bill, but after being purchased by the hobbits Bill became a much happier pony. He was given the name 'Bill' by Sam Gamgee shortly after the party left Bree.
After arriving in Rivendell, Bill became acquainted with other ponies, and this influence resulted in a good deal of self-improvement. He left Rivendell a much wiser pony, not to mention healthier and happier.
He accompanied the Fellowship of the Ring from Rivendell to the doors of Moria, but had to be left behind there because the company could not take a pony through the mines of Moria. But being a wise pony by this point, he managed to survive on his own and travelled back to Bree, where he eventually hooked up again with Sam on his return trip.
See also Middle-earth