Mete Han and Atilla were both Hun emperors and feature on the statue.
We’re going to consider the heads in chronological order and today start with the earliest two:
Mete Han and Atilla, both Hun emperors.
Mete Han founded the huge Asian Hun Empire. The Huns were a Turkic race and modern Turks consider Mete Han to be their earliest recorded ancestor. He reigned from 209 – 174 BC.
Atilla who lived from 406 – 453 according to Wikipedia (his statue claims he was born in 400) was king of the Huns from 434 until his death.
He was leader of the Hunnic Empire which stretched from Germany to the Ural River and from the Danube River to the Baltic Sea.
During his rule he was one of the most fearsome of the Western and Eastern Roman Empires’ enemies: he invaded the Balkans twice, he marched through Gaul (modern day France) as far as Orleans before being defeated at the Battle of Chalons; and he drove the western emperor Valentinian III from his capital at Ravenna in 452.
He reached Constantinople and Rome but refrained from attacking either city.