According to the Online Etymology Dictionary:
mannequin: 1902, “model to display clothes,” from Fr. mannequin (15c.), from Du. manneken (see manikin). A French form of the same word that yielded manikin, and sometimes mannequin was used in English in a sense “artificial man” (especially in translations of Hugo). Originally of persons, in a sense where we might use “model.”
“A mannequin is a good-looking, admirably formed young lady, whose mission is to dress herself in her employer’s latest “creations,” and to impart to them the grace which only perfect forms can give… [“The Bystander,” Aug. 15, 1906]
Later (by 1939) of artificial model figures to display clothing.
Mannequin Heads, Greneker Display Company, Los Angeles, CA, USA, 2003. Copyright Lorna Roth.
See the collection of photos of mannequins in the website image gallery as assembled by Roth over the course of her research around the world.