Prince Cantacuzene was indeed a horseman. His position would have been equal to the British monarchy's Master of the Horse in The Queen Mother's court.
The Cantacuzene's originally came for Byzantium and a couple of them were Emperors there. Not always the safest position in that corrupt empire.
After 1453, when Constantinople fell to the Ottoman Turks, the Cantacuzenes hightailed it out of there and went to Rome before settling in Rumania - there are still Cantacuzenes there today. The Cantacuzene Palace in Bucharest is a sight to behold - very ornate, mosaic and paraquet floors, both gold and painted ceilings and with several classical & baroque & roccoco rooms...
I'm sure they must have stripped the Constantinople treasury before they left.
A minor branch of them left and fought for & supported Peter the Great in Russia; and so were ennobled to Prince. But, in the Russian societal strata, I don't think they were as high up as many other Russian noble families....
They were simply loyal to the monarch but not too prominent really.
However, this did not prevent Nell from catawalling the title "Princess" once she returned to the USA.
I asked her once why some people still called her Princess Eleanor; and she said matter of factly that when she came back to NYC in 1936, the newspapers wrote that Princess Eleanor Romanov had returned and "I simply didn't correct them!!"... And, she never did....