This head is a portrait, but not of a particular individual. It represents a type of person living on the outer fringes of the Roman Empire; Dacia was roughly where Romania is now. The Romans portrayed these people with certain physical characteristics, such as long hair for women, as seen here.
The sculpture dates from the time when the emperor was Trajan, who fought and won many wars with the Dacians and other tribes of northern and eastern Europe
St Petersburg, Hermitage 358
Waldhauer: Die Antiken Skulpturen der Ermitage, III, 83, no.353, pl. XLIV