Legatus Legionis. The Legatus was the most respected military officer in Rome. A Legatus held full power of his legion.
Romanos durante el Alto Imperio, por Leo Karpacz. Más en www.elgrancapitan.org/foro
This kind of armour, the Lorica Segmentata was not worn by cavalry. It was a heavy infantry armour, incompatible with cavalry combat. Same thing for the helmet. Cool painting, though, although the man has a tiny head.