This is a rare type for Constantine, though it has the same reverse as the more common ADVENTVS type. This coin "looks to Constantine as Rome's hope of salvation" from Maxentius. (RIC VI pg 121)
Constantine I
A.D. 311- 312
CONSTANTINVS P F AVG; laureate and cuirassed bust right.
SPES REIPVBL; Prince riding left, r. raised, l. holding up spear, on horse pawing seated captive to left, * in right field.
in ex. PLN
RIC VI London 241