From what I can tell you probably have a Roman coin and with the cracked planchet odds are good it's the real thing....so you're looking at a coin at least 1,500 years old.
Maybe one of the coin experts can narrow it down as to what Emperor it is.
Obv: DN CONSTANTIVS IVN (NOB C), Bare headed, draped, cuirassed bust facing right, [delta] behind head. Rev: FEL TEMP (REPARATIO), Soldier spearing fallen horseman;
Note: The horse is still standing more erect than in most issues of this type the horse in down on its knees or lower.
Don.......
PS: I agree with "villagenut" as to Constantius Gallus (351-354 AD),
Just to help on Villagenut’s correct ID of a CONSTANTIUS GALLUS coin as it will not be found on the Wildwind’s site being a rarer variety.
The coin obverse (Front) inscription is DN CO[NSTANT]IVS IVN NOB C Centenionalis type AD 351-353 bare head right with ∆ behind. Reverse (Back) inscription is FEL TEMP REPARATIO (return of good times) Soldier spearing fallen horseman with •S• in left field. In the exergue (base of reverse) is the mint mark of [A]SIRM. This is a SIRMIUM (now Sremska Mitrovica, Balkins) minted second series.
Reference is Roman Imperial Coinage Volume VIII page 387 Sirmium 46.