Denarius. 19-18 mm. 3.09 grams. 6:00 die axis.
Trajan, 98-117. Struck 107-111 [Hill]

IMP TRAIANO AVG GER DAC PM TRP, laureate head right
DANVVIVS is exergue, COS V PP SPQR OPTIMO PRINC, Danube river god reclining on rocks, cloak floating above, his right holding a ship's prow.

RIC 100. Hill 472 "107". Sear II 3138.

Trajan conquered Dacia which was the first territory across the Danube River added to the empire.  Formerly, and after Aurelian who relinquished Dacia, the mighty Danube was the boundary of the Roman Empire. 

For a wonderful writeup of this type, with lots of pictures of the relevant part of the Danube, see:

https://www.cointalk.com/threads/trajan-the-best-emperor-and-the-mystery-of-the-river-god.318843/

Return to page 2 on reverse types unique to particular emperors.