Clarence

In the 14th century a son of Edward III was married to the heiress of Clare in Suffolk. The title of Duke of Clarence was created for him, and has been held by various British princes in the years since. Clarence began to be used as a given name in honor of one of these, Prince Albert Victor of Wales, who was the popular eldest son of Edward VII, and became Duke of Clarence in 1890. Clarence was an extremely popular name from the late 19th century into the early decades of the 20th century. It remained in the top 100 in the U.S. until the 1950's, but has declined slowly in popularity since then. Today it is still in the top 1000, and hasn't quite become "rare," but because it was so heavily used in early decades, it's considered a bit outdated. Since Clarence is a relatively new entrant to the name game, it is anyone's guess whether it will ever regain any of its popularity of the early 20th century, or if it will turn out to have been a "trend" of its time.