There are two manuscripts dated from 1594 and 1603, that granted the Lateran chapter six and then five ounces from the Aqua Felice under Clement VIII. The gift, although unable to find the manuscripts, was almost certainly on the condition that the chapter would build a pubic fountain, at its own expense. Because it was built for a private organization it is basically impossible to find any documentation about the building of the fountain.
There are, luckily, two documents that suggest the fountain had been started, at the very least, under Clement VIII. One document comes from a private citizen on April 3, 1064 detailing a purchase of water from the conduit and fountain at St. John Lateran. The other document is the fountain, bearing stars and embattled device of the Aldobrandini, the family of Clement VIII. The canons had this coat of arms carved in it, in commemoration of the gift.
The added fleur-de-lis on each side of the statue was a contribution from Leo XI, who only resigned for 27 days. It would be Paul V who made the final contributions to the fountain. In 1607 he added the heraldic dragon emblems, making them so large, that they take up all the decorative space available.
So what happened to the vanished statue of St. John? Leo XI probably had is placed on the pediment, and writers, are no doubt correct, when they attribute it to Taddeo Landini, who also sculpted the four young men on the Tortoise Fountain. No one knows what happened to the original, but the one made by Taddeo was placed in the nearby St. John’s Baptistery, where it remains today!