Mithradates Eupator Dionysos (120-63 B.C.) was the last Hellenistic ruler of the Kingdom of Pontus on the Black Sea coast of Anatolia. He is renowned for the three wars fought against Rome in the early to mid 1st century B.C., falling eventually to the Roman general Pompey the Great.
Mithradates spent the first part of his career expanding his kingdom, encroaching on neighboring territories. He moved the seat of his kingdom to Pergamon and was the last Greek ruler to reside there.
A passionate philhellene, Mithradates identified himself with Alexander the Great. This accounts for the style and stance of this bronze figure, relating closely to Lyssipan depictions of Alexander, such as the bronze in the Harvard University Art Museum, no. 38, p. 118 in Yalouris, et al., The Search for Alexander, an Exhibition, thought to represent the now-lost Lyssipan Alexander with a Lance. Like Alexander, Mithradates is shown here likely once holding the lance in his left hand.
For a marble head of Mithradates with similar wildly touseled tresses, see no. 84, pl. 52,3-4 in Smith, Hellenistic Royal Portraits, and on coinage, figs. 207-209 in Davis and Kraay, THe Hellenistic Kingdoms, Portrait Coins and History.