Possibly Presian or Zvinitsa (unclear)

"The De Administrando Imperio of Konstantinos Porphyrogenetos names "Michael Borises Bulgariæ princeps" and his "pater Presiam". Boris was the son of Swiniza according to Theophylact of Ohrid."

• Source - http://fmg.ac/Projects/MedLands/BULGARIA.htm#GeorgiTerterdied1304

For more information see The Early Medieval Balkans: A Critical Survey from the Sixth to the Late Twelfth Century by John V. A. Fine, page 108-9.