Order of the Silver Serpent
Every kingdom needs its lead warriors, its Knights, to guard the rulers and defeat enemies of the state on the field of battle. Britannia's Knights were initially known as the Order of the Silver Serpent.
The Order was often found in Ultima Prime, the single-player games upon which UO is based. Known members included Geoffrey and Dupre. The Order derived their name from the Silver Serpent, the symbol of the realm. The Order was traditionally housed, and trained, in Serpent's Hold.
Eventually, for reasons that were never quite clear (though there were hints that it was some kind of copyright issue), the Order's name had to be changed to the Royal Knights. Eventually it was called the Royal Britannian Guard.