Mark Twain wrote in a letter, I'm not sure to who: "I was named Langhorne for a valued friend of my father, but he was not a relative, but a comrade of my father's youth in Virginia.I merely served by my name as a reminder of that loved and lost comradeship of a vanished day."I got this from Elizabeth Appleton's notes, but I have seen it in print as well. She concluded that Henry S. Langhorne was most likely the friend, who spent his orphan days with his aunt Mary Langhorne Calloway, probably in Cumberland and Buckingham, and later in Bedford counties.I think Twain's mother thought he had originally been named Lampton after her brother(?), but the name had been somehow changed later. Of course, those two versions are not incompatible.