Fontana dei Quattro Fiumi

The Fontana dei Quattro Fiumi is just a short way from the distinctive doughnut shape of the Pantheon.

Fontana dei Quattro Fiumi is the Fountain of the Four Rivers.  It is not really ancient, as it was built in the 17th century AD, and it was a sign of the spread of Roman Catholicism.  The four river gods depicted represented the four major rivers of the four continents under Papal authority.

God of the Ganges, representingAsia.  The Ganges is in India but, although India is best known for being colonized by the Protestant English, they did not kick out the Catholic French decisively until a hundred years or so until after this fountain was built.  The Jesuits, a Roman Catholic religious order, had had a significant and influential presence in Japan  a hundred years before the fountain was built, but, by this time, Emperor Hideyoshi had adopted an extremely seclusionist foreign policy and had kicked out or killed all foreign missionaries and local converts.

God of the Nile, representing Africa.  His head is covered because, at the time, no one knew where the source of the Niles was located.  It was found in 1875 by Henry Morton Stanley, a British explorer, perhaps even more famous for saying "Dr. Livingstone, I presume?" on finding the missionary David Livingstone in the middle of Africa.  If you notice the band around his left leg, I assume that it is a symbol of subservience in some way.

The God of the Danube, for Europe. 

The God of the Rio de la Plata, for the Americas.  It is significant that this river is in South America, which was a stronghold of the Spanish and Portuguese, and not the much longer Mississippi River in North America.  By the time this fountain was designed, Catholics (especially the French, and Jesuit missionaries) had been using the Mississippi as a prime trade and communication route for over a century and had already traced the Mississippi at least five times as far as the Rio de la Plata.  (The Mississippi River is 2,530 miles and long and the Rio de la Plata is 180 miles long.)  I assume that the choice of rivers was determined primarily because of the greater trade value of South America (gold, instead of fur) but also because by this time, the Protestant English and the Protestant Dutch had a pretty significant stranglehold on the Atlantic seacoast, and were a significant threat to Catholic control of North America).
If you notice the right ankle of this fellow, this is almost certainly a symbol of slavery and the shaved head also seems symbolic of something.  (A lot of captions said that this river god is symbolized by an African slave.)

 (All pictures are from Wikimedia Commons.)