As it turned out, the powerful bodies of the early Friesian horses had inspired the robust aesthetic of the Horses of St. Mark’s in Venice. According to the Web site, the Horses of St. Mark’s were so beautiful that they had become “history’s most frequently stolen pieces of art.”

Langdon had always believed that this dubious honor belonged to the Ghent Altarpiece and paid a quick visit to the ARCA Web site to confirm his theory. The Association for Research into Crimes Against Art offered no definitive ranking, but they did offer a concise history of the sculptures’ troubled life as a target of pillage and plunder.

This appears to be a reference to the blog post(s) by Judge ArthurTompkins, an ARCA Lecturer, written in June of 2011: The Four Horses of the Basilica San Marco, Venice (Part I); More on the History (Part II); Continued Short History (Part III); and The Four Horses Rest Inside St. Mark's Basilica in Venice After Being Plundered from Constantinople in the 13th Century (Part IV). On the ARCA blog is another post about the "The Triamphal Quadriga" in Paris Diary: Replica of Stolen Art at Arc de Triomphe du Carrousel.

Then Brown is a bit more definitive about what academics would question:

The four copper horses had been cast in the fourth century by an unknown Greek sculptor on the island of Chios, where they remained until Theodosius II whisked them off to Constantinople for display at the Hippodrome. Then, using the Fourth Crusade, when Venetian forces sacked Constantinople, the ruling doge demanded the four precious statues be transported via ship all the way back to Venice, a nearly impossible feat because of their size and weight. The horses arrived in Venice in 1254, and were installed in front of the façade of St. Mark’s Cathedral.

More than half a millennium later, in 1797, Napoleon conquered Venice and took the horses for himself. They were transported to Paris and prominently displayed atop the Arc de Triomphe. Finally, in 1815, following Napoleon’s defeat at Waterloo and his exile, the horses were winched down from the Arc de Triomphe and shipped on a barge back to Venice, where they were reinstalled on the front balcony of St. Mark’s Basilica.

Although Langdon had been fairly familiar with the history of the horses, the ARCA site contained a passage that startled him.

The decorative collars were added to the horses’ necks in 1204 by the Venetians to conceal where the heads had been severed to facilitated their transportation by ship from Constantinople to Venice.