Yourcenar's Hadrian is ennumerating some of the significant art-works that he has collected.
The Hermaphrodite is a sculpture, originally in bronze and by the Hellenistic sculptor, Polycles. Whilst this original is lost, copies survive, most notably the "Borghese Hermaphrodite," now in the Louvre.
The Centaur (actually there are two, the "Old Centaur" and the "Young Centaur") refers to the Capitoline Centaurs, discovered by Monsignor Alessandro Furietti at Tivoli in 1736, and now in the Capitoline Museum in Rome.
The Niobid is the "dying Niobid," found in the Gardens of Sallust in Rome, and now at the Palazzo Massimo.
The Venus is a copy of Praxiteles's Aphrodite of Cnidos, versions of which are to be found in the Vatican Museum and the National Museum of Rome (Ludovisi Collection).