The Batavians were one of the best men Rome could draw in their Auxiliary cavalry corp. They were known to be super tough and capable fighting men from the Geramnic tribes of the Batavii region. The tribal name, probably a derivation from batawjō ("good island", from Germanic bat- "good, excellent" and awjō "island, land near water")
The Batavians were first recorded by Julius Caesar.
“The Maas rises from mount Le Vosge, which is in the territories of the Lingones; and, having received a branch of the Rhine, which is called the Waal, forms the island of the Batavi, and not more than eighty miles from it falls into the ocean. But the Rhine takes its course among the Lepontii, who inhabit the Alps, and is carried with a rapid current for a long distance through the territories of the Sarunates, Helvetii, Sequani, Mediomatrici, Tribuci, and Treviri, and when it approaches the ocean, divides into several branches; and, having formed many and extensive islands, a great part of which are inhabited by savage and barbarous nations (of whom there are some who are supposed to live on fish and the eggs of sea-fowl), flows into the ocean by several mouths.”
The served as regular units the reign of Augustus and Tiberius in his campaigns against Arminus. They were known to be very well swimmers and guerilla fighters. They also bodyguarded the emperor in his own bodyguard unit "the Equite Singualres" that operated alongside the Praetorian Guard.
The Batavians according to Tacitus could cross the Rhine on their horses without losing formation.
The Bataivans were recorded to be used as an Imphibous shock unit like today's SEALs. Tacitcs writes in the Battle of Idivisito how the Batavians sneaked acoss the river causing the enemy Germans to detatch a formation to intercept them. The Batavians formed a shieldwall and fought heroically until the Roman cavalry saved them.
The emperor Claudius employed the Batavians in his invasion of Britain. Dio Cassius describes their actions during the battle of the River Medway in 43 AD
“The barbarians thought that Romans would not be able to cross [the Medway] without a bridge, and consequently bivouacked in rather careless fashion on the opposite bank; but he sent across a detachment of Germanic tribesmen, who were accustomed to swim easily in full armour across the most turbulent streams. [...] Thence the Britons retired to the river Thames at a point near where it empties into the ocean and at flood-tide forms a lake. This they easily crossed because they knew where the firm ground and the easy passages in this region were to be found; but the Romans in attempting to follow them were not so successful. However, the Germans swam across again and some others got over by a bridge a little way up-stream, after which they assailed the barbarians from several sides at once and cut down many of them.”
The Gerneral Suetonious Paulanus also used them I believe.
However the Batavians later revolted and were defeated by Rome. Rome posed harsh terms by making Legio X Guard them. They still served under the Auxilia and were recorded to have been serving to the 4th century under Constanitius. However their name would never be well recorded after the revolt.
I hope they are in the game and a replacement for Praetorian cavalry. Praetorian cavalry I believe would not have been numerous.