This unit first appeared in Britain in the train of emperor Hadrian during his historic visit to the island in 122AD. They are the most frequently attested regiment at Maryport on the Cumbrian coast, probably the first unit to occupy the fort, where they are first mentioned on an altar dated 123-38AD and on several other undated stones and tiles. Of the twenty-three altars to Jupiter found at Maryport, no less than ten are inscribed with the name of this regiment and a further six were dedicated by men known to have commanded the unit, which likely indicates that they were stationed there for a substantial period. They are recorded on a single tombstone at Ardoch in Tayside dated to the period 139-61AD, and also at Netherby in Cumbria on four inscriptions dating between 213-222AD. The Notitia Dignitatum indicates that by the turn of the fifth century, the unit had been moved on to Stanwix, also in Cumbria, where they shared the fort with the Ala Petriana.
Cohors I Hispanorum equitata was originally a five-hundred strong part-mounted regiment levied from amongst the many tribes of the imperial Spanish provinces. It is very likely that they were increased in strength to a nominal one-thousand soldiers at sometime during the second century, at which time their name was changed to Cohors I Aelia Hispanorum milliaria equitata. The Aelia title indicates that this probably occurred during the time of the Aelian emperors, the last one of which was Antoninus Pius who died in 161AD.