Monument of Aemilius Paullus was erected in the Sanctuary of Apollo at Delphi shortly after 167 BCE in order to commemorate the Roman victory at the Battle of Pydna over King Perseus of Macedon.
— David Gibbins: "Destroy Carthage: The Triumph Of Aemilius Paullus", 2013.
— Jeremiah B. McCall: "The Cavalry Of The Roman Republic. Cavalry combat and elite reputations in the middle and late Republic", Routledge: London, New York, 2002.
— Roman mausoleum of Julii at Glanum, north face (1st century BC) (Glanum)
Interesting then how this looked on the Gundestrup cauldron (richly decorated silver vessel, thought to date [… ] or more narrowly between 150 BC and 1 BC):
— M.C. Bishop: "Cavalry Equipment of the Roman Army in the First Century A.D.", in: J.C. Coulston (ed.): "Military Equipment and the Identity of Roman Soldiers. Proceedings of the Fourth Roman Military Equipment Conference", BAR International Series 394, Oxford, 1988. (PDF)
Since I gather that not only strictly military depictions in full armor are requested:
The front of an alabaster cinerary urn: horsemen and musicians arrive at a tomb. Date 100BC (circa). British Museum number 1925,1218.1 equestrian parade; lictor with fasces visible top left; right, preparations for sacrifice, with victimarius holding ram or sheep and flute player and lyre player at podium temple (triton? giant? in pediment). London, British Museum. (Photo from DAIR 40.817.)
Of specia interest will be the slightly fuzzy dated, but certainly at least touching the timeframe
The "Bronzi Dorati da Cartoceto di Pergola" is the only group of gilded bronzes in the world dating from Ancient Roman times still in existence. They most likely represent a family group, originally two female figures, cloaked and veiled, and two horsemen in high-ranking military garb with richly ornamented horses.
Based on the style, the statues are thought to be from the late Republican age, probably the 1st century BC or 1st century AD.
— Museum of Roman Gilded Bronzes. Rare Ancient Roman Life-Size Statues
There are quite a few Roman copies of Greek sculptures, for which the 'originals' have to be dated quite early. Some other sculptures and reliefs are dated with uncertainties, or sometimes just 'confusingly'. For example, the monument tomb of Tiberius Flavius Miccalus is on the web almost always given as 1st century BC,
but the supposedly more reliable Arachne database lists it as 1st century AD: