The great thing about the Civil War, as a subject of history, is that there is so much material available for research, all of it in English, and a lot of it online.
Which, of course brings us, as all historical inquiries tend eventually to do, to the name of Sheridan's favorite horse. Perhaps you studied this perplexing matter in school?
I had long ago heard that the equine Rienzi was named after a town in Mississippi from which it was taken but but was suspicious about this. Why would that state, recently a frontier, have such a name in the 1860s?
I knew that Wagner's opera Rienzi had toured the U.S. before the Civil War. Was Sheridan an opera-goer? (Hitler had adored Rienzi, especially the scene where everyone is giving the Roman salute and shouting, "Heil, Rienzi!") The likelihood of Sheridan having been overwhelmed by a German-language opera seemed remote, however. The likelihood of a rural hamlet in Mississippi choosing an operatic name was odder still.
Then there was the novel, Rienzi, The Last of the Roman Tribunes on which the opera was based. Its author, Sir Edward Bulwer Lytton, had a runaway best seller in 1834: The Last Days of Pompeii. Rienzi, the book, was the eagerly awaited follow-on published in 1835.
Could Rienzi the town have been named after Rienzi the novel? The town website says it was founded in 1836. This seems to crowd 1835 a little to closely. And the website says it was named after the historical figure, without reference to Lytton's novel.
So the question is whether there was a general craze for Rienzi, the historical figure, in the pop culture such that both Lytton and some village elders in Mississippi could have tapped into that at almost the same time.
Hmm. The other great thing about the Civil War, as a subject of history, is that you rarely get to the bottom of things ... even something as simple as the name of a horse.