Lucius Aelius Sejanus rose to power under the reign of the Emperor Tiberius, the adopted son of Augustus. Sejanus was the first, but not the last, to exploit his position of Commander of the Guard to pursue his own ambitions.
He made extensive efforts to ingratiate himself with the Emperor while at the same time quietly eliminating any others including Tiberius' own family, from having any connection with the Emperor himself.
Inevitably, he over-reached himself by trying to get Tiberius to make him his 'heir apparent'.
This led to his downfall and execution but for 17 years he led the Guard and was a major power in Rome itself.
He is most notably portrayed in the classic television series, "I Claudius' Our mounted figure shows him at the pinnacle of his power.