One of my ancestors was named Twaddle. Well--more than one, of course.
Rare names have a tendancy to disappear for various reasons. For one thing, the rare names tend to be silly, embarassing or even offensive. Not many Hitlers in New York City nowadays but there were quite a few in 1939.
A fun site if you are interested in names is the US Surname distribution maps at Hamrick software:
http://www.hamrick.com/names/
You can search for a name in several U.S. Census data sets and even watch how the distribution changes over time. Sometimes this will tell you a lot about history, family history, patterns of settlement. If the name is very common, of course, you get a complete wash of every state.
Take for example, the Dorset pioneer, John Gallup. He arrived in the U.S. in 1630 (Massachusetts) and became the ancestor of millions of Americans and Canadians (plus many others), including the founder of the Gallup poll, President George Bush, and the railroad pallroll master after whom Gallop, New Mexico is named. (GWB is my tenth cousin, once removed.)
If you compare the maps with the genealogy websites, you will find that the maps constitute a decent four frame movie of the history and distribution of Gallup descendants, who are everywhere.
The biggest concentration of the US Twaddles were in New Hampshire in 1880. This makes sense--New Hampshire is notoriously eccentric and tolerant--and the original Twaddle pioneers probably came into the continent via Boston or New York.
McCoy is a famous Appalachian name--the first map (1850) is very interesting, however. Why an arc? Were the McCoys natural born colonizers who moved out of the South in an arc by 1850? Or were they run out of the Southern lowlands?
