It was said that the Fellowship was comprised of nine members: two men, four hobbits, one wizard, one elf, and one dwarf. The story goes that they battled their way through all obstacles to defeat Sauron. There is much truth to this, yet the storytellers are all liars by omission. There were not nine Fellowship members. There were ten. The tenth member was a silent one, bearing the burden of their quest on his strong back, plodding along for there was nowhere else to go. He was not hand picked by Elrond, but the Fellowship would have been lost without him.
His name was Bill, and his past is irrelevant. He was a gaunt pony when they found him, however, and Sam saw through the dirt, skin and bones and saw something he would be hard-pressed to find later on his journey. He saw a friend, and potential. The pony loved Sam, and was fiercely protective of the small hobbit. Most disagree, and say it was simply instinct, but they are wrong. It was not instinct. Bill chose to follow out of love for his master and out of obligation, a life-debt of sorts.
Bill was betrayed along the road. He was angry when Sam took off his bridle and allowed Strider to send him home. But he didn't know where home was. Home was wherever Sam had gone off to and now he was free, but free wasn't ideal. He was lonely, no one was ever around to rub his nose or feed him a spare apple and there wasn't much food on the mountainsides or on the road to Moria. He trotted on aimlessly for days before returning to Bree, but simply because of need. His stomach was empty and he was close to death. But he survived.
He wanted to see his Sam again.