It's my understanding that while the mountain dulcimer existed, it was largely unknown outside Appalachia until after the Civil War. However the hammered dulcimer was a popular parlor instrument. At least that's what I've read.

A small mountain dulcimer would not be any more bulky to carry than a fiddle and there is documentation for soldiers having those. Banjer, if your grandpappy was from the mountains of Kentucky, you may well have been familiar with the mountain dulcimer. :-)