The drone horn is basically a large, straight pipe (like a large, straight bugle). It has no fingerholes, and produces a drone-like tone on the same pitch. The horn, in the form of a hollow pipe-like object, such as a piece of hollow reed or bamboo stem, must be one the oldest manufactured air instruments.
As the horn is much larger than a bugle - both in diameter and length - its tones are on a lower frequency than those of a bugle, but its acoustic structures are similar. Many cultures have this instrument, known by local names.
It is very likely a more recent development than, for example, the flute, as the materials used for its construction involves a more complicated manufacturing process than simply drilling holes into a piece of bamboo or reed or bone to make a flute.
Source: http://www.eliznik.org.uk/RomaniaMusic/images/bucium-2.htm
Source: http://en.wikipedia.org/wiki/File:Alphorn-MJ.jpg
Source:
http://www.musiccentre.ca/influences/instrument.cfm?type=karnay http://www.sairamtourism.com/uzbekistan/musical-instruments
Source:
http://www.thevuvuzeladiaries.com/tag/background/
This is the instrument typically used by Buddhist monks.