No twin brother, but a twin sister, I am lead to believe.
Speaking of Bill the Pony's remarkable survival in the Wild, I am fairly impressed. Although I don't believe that The Watcher in the Water was interested in the Pony, despite his tender flesh.
This is more of interest to the Wargs of the area. Rest assured, Bill the Pony could not survive an assault by one Warg, let alone a pack of Wargs. Thus, I must assume that the Wargs actually helped Bill the Pony, or at least did not hinder him.
For more information about Wargs, you know who to ask.
But the relevant point I am trying to get across here is that Bill the Pony was not a remarkable Pony, but a normal beast of burden. Thus I doubt he would have killed Bill Ferny with that kick.