The harp guitar is one of the few oddball hybrid instruments to stand the test of time. Depending on when you mark the first occurence of a “true” harp guitar, it’s been around in one form or another since the 18th/19th century (also in early forms as the “harp lute”).

For more info:
harpguitars.net – As its name suggests, a site dedicated to harp guitars. Includes explorations into the instrument’s history and the entymology of the term “harp guitar”.
myspace.com/andymckee2 – More music by Andy McKee. See also Andy’s profile at Candy Rat records.