X:1 T:Harlequin Neptune M:C L:1/8 R:Country Dance B:Thompson's 24 Country Dances for 1781 B:Samuel, Ann & Peter Thompson -- Compleat Collection of 200 B:Favourite Country Dances, vol. 5 (1788, No. 178, p. 87) Z:AK/Fiddler's Companion K:D dAFD dAFD|dAFD E2E2|dAFD EFGA|BGFE D2D2:| |:e|fdfa gecA|fdfa g2g2|fdfa gbeg|fedc d2d2:|]
HARLEQUIN NEPTUNE. AKA and see "Neptune (2)," "Pretty Girl (2) (The)." English, Country Dance Tune (4/4 time). D Major. Standard tuning (fiddle). AABB. In addition to the printing by London publishers Samuel, Ann and Peter Thompson in their Twenty-Four Country Dances for the Year 1781, the tune appeared in Longman, Lukey & Broderip's Bride's Favorite Collection of Two Hundred Select Country Dances (London, 1775, p. 89).
King of the genii: or, Harlequin Neptune was a 1775 stage pantomime, "with music and dancing" by English composer Charles Dibdin (1745-1814), mentioned in Sadler's Wells advertisements. It was one of many such productions in which the character of Harlequin is the protagonist, playing alongside another stock character, the clown. There were so many comic stage plays written for Harlequin that they form their own genre, Harliquinades 
The melody appears in the c. 1837-1940 music manuscript of Shropshire musician John Moore as "Pretty Girl (2) (The)," and by poet and musician John Clare as "Neptune (2)."