Garden Water Fountains of Rome
Before dealing with the two fountains at the sides of Piazza Navona, I would like to say a few words about a very modest but very small fountain that used to exist and which disappeared, I'm not quite sure when, but probably soon after 1870. This was the fountain in Via della Scrofa ["scrofa" means sow], where the carving of the sow can still be seen today, at number 94. In 1832, water still ran from the fountain into a small basin beneath, but as this was in the way of pedestrians, the water and the basin were moved a few yards to what is now the corner with Via dei Portoghesi, but the sow remained in its old place, albeit with a dry spout. Despite centuries of corrosion, this little bas-relief sow (approx. 50cm long) was almost certainly a votive offering, a sculpture made to bring prosperity to whoever dedicated it. At least, that was religious meaning of the sow to the Ancient Romans; for instance, in the myth retold in Virgil's Aeneid, the hero Aeneas, after landing on the banks of Latium, meets with a pure white sow, the symbol of prosperity and power (i.e. Rome, the famous city which Aeneas was supposed to have founded). However, unless I am mistaken, no enthusiastic archaeologist has yet made a study of this carving.
There was an inn called "The Sow" on the site of this fountain
This apart - which in any case you probably know as well as I do - I believe this carving must have existed more or less in the same place as it is now, at least since 1445, when there was an inn called "The Sow" on this site. Also, in 1534, a motu proprio [administrative edict] by Pope Julius III referred to the "street known as Sow [Street]" in relation to a matter concerning the butchers, but I won't go into it here because it has nothing to do with the fountain. However, we can be sure that the name given to the street dates back much further than the years I've just mentioned. Although Cassio (I, 289) thought that the ancient fountain carving was adapted for use as a fountain as part of the water fountain scheme implemented by Gregory XIII, this is definitely not the case. The State Archives hold a very precise and detailed list of every fountain built by the popes in the last 25 years of the 16th century (the fountains set up in all parts of the city to which the new 
Aqua Vergine could be brought) and this fountain is not included. On the contrary, the same list shows that on 22 February 1595, by a document in his own handwriting (which unfortunately I have been unable to trace), Clement VIII (Aldobrandini) granted to a certain Raffaele Casali one and a half ounces of Aqua Vergine "for his house in Sow [Street] and for the public fountain"; it may have been a great concession to Mr Casali, but he was "obliged to make the said public fountain" - and I believe it to be the one in Via della Scrofa. Besides, it was the custom - as seen in other cases - for the Papal and City Councils to grant small quantities of water to private citizens on condition that, in return, they should erect a fountain, or pave a street near an existing fountain, or dig a drain. To explain this odd but practical custom in more detail - and in case today's City Council should think of renewing it to solve a few problems - I will quote from something Fea wrote in 1833: "If these [public] fountains belong to the houses or other buildings then the owners have an implied duty which they cannot alter without the agreement of the respective Higher Courts.
The semi-public [fountains] are the responsibility of the owners of the houses, or boundary walls if large estates, because they are intended for the public who have irrevocable rights and possession thereof, on the grounds that, moreover, originally a greater quantity of water was given to them [the property owners] free of charge, or for less than the usual price, on condition that they continue to provide and maintain these fountains. Furthermore, the [property] owners pay no taxes for this water and enjoy the "return" on that same water". You may well already have imagined the countless number of strict regulations that were posted on the walls of Rome ordering "all and every person of whatever state, rank or condition, including ecclesiastics, and however privileged or highly privileged" not to take for themselves all the water from the public fountain entrusted to their loving care, but only their legitimate share; otherwise - the regulations threatened - offenders would run the risk of penalties amounting to enormous quantities of scudi and, it goes without saying, "corporal punishment at our discretion, according to the type of person". Well, what else would you expect?