Ancient: strator: an officer on the staff of a legatus

Legatus

A legatus was a general in the Roman army, equivalent to a modern general officer. Being of senatorial rank, his immediate superior was the dux, and he outranked all military tribunes...

, a Roman provincial governor. His precise duties are uncertain, but must have had something to do with the procurement and supply of horses, as the word is derived from Lat. sternere (to strew i.e. hay, straw). He may thus have been in charge of a mansio

Mansio

In the Roman Empire, a mansio was an official stopping place on a Roman road, or via, maintained by the central government for the use of officials and those on official business whilst travelling.-Background:The roads which traversed the Ancient World, were later surveyed,...

, a Roman provincial way-station. The word occurs on an undated slab from a Roman tombstone, found at Irchester (Northants., UK)

Mediaeval: A strator is typically a squire

Squire

The English word squire is a shortened version of the word Esquire, from the Old French , itself derived from the Late Latin , in medieval or Old English a scutifer. The Classical Latin equivalent was , "arms bearer"...

whose task consists of holding the bridles of his prince's horse, and of assisting his master when the latter mounts or dismounts his horse.