Here is a very common type...but not as common with a horseman on the shield. This type is mentioned in the footnotes of RIC. Crispus A.D. 320 19x20mm 2.6g IVL CRISPVS NOB CAES; Laureate and cuirassed bust left, spear pointing forward, shield with horseman on left arm. VIRTVS EXERCIT; Standard inscribed VOT/X with captive seated on ground on either side, in left field S, in right field F over HL. in ex. ΓSIS ✶ RIC VII Siscia 123
This Crispus has a star on the shield which is rarer than the horseman on shield...slight double strike on obverse with a few issues including obverse legend ending with CAESS. Crispus A.D. 320 19x20mm 2.6g IVL CRISPVS NOB CAESS; Laureate and cuirassed bust left, spear pointing forward, shield with ✶ on left arm. VIRTVS EXERCIT; Standard inscribed VOT/X with captive seated on ground on either side, in left field S, in right field F over HL. in ex. ASIS ✶ RIC VII Siscia 123
I have a coin of his dad with a similar looking shield Constantine I Silvered Follis 318-319 AD Siscia Obv Bust left helmeted draped and cuirassed holding in his left hand shield emblazoned with charging horseman riding over two prostrate enemies. A spear is carried over his right shoulder Rv Altar flanked by two Victories supporting a shield between them. RIC 56var 2.85 grms 18 mm Photo by W. Hansen This coin proceeds the two coins cited above by a year or two. The charging horseman motif on an Imperial shield is not new. I have seen this device on an aurelianus of Probus from the mints of Rome Serdica and Cyzicus. There are probably others. On a personal note: I bought this coin in a NAC auction in 2011. It was remaindered. I guess no one loved it.
another tough one to find--a hippocamp on a Siscian VLPP from Constantine Constantine I A.D. 318-19 18mm 2.4gm IMP CONSTANT-INVS AVG; high crested helmet, cuir., spear across r. shoulder, shield on left arm inscribed with hippocamp. VICTORIAE LAETAE PRINC PERP; two Victories stg., facing one another, together holding wreath [shield] inscribed VOT PR on altar. in ex. ΔSIS RIC VII Siscia 56