What are agraulos?

Some articles on agraulos:

... Aglaulus or Agraulos (Ἄγραυλος) is a name attributed to three figures in Greek mythology ... commonly), Aglaurus, Aglaulos, Agraulus, Agravlos, or Agraulos ... Agraulos ("countryside flute") was probably the original form of the name, with the r and l commonly switched to produce the prevalent Aglauros form ...