Peter, as y'all know, was responsible for the Gospel we call "Mark." The tradition is solid: Mark simply served as Peter's stenographer as Peter gave a series of talks before the "equites" of Rome. I'm often asked, "Who were these equites?" So let me try and answer that question here.
Generally speaking, Roman citizens fell into two basic classes. There were the patricians who could trace their ancestry to the paters -- the founders of Rome. Then there were the plebians -- the everyday people or what we might call the "non-patricians." One subdivision of the plebians was a group of people called the equites, or the "equestrian" class. This class was comprised mostly of wealthy businessmen. The equites were originally those rich enough to afford their own horses when they fought in the army (i.e., cavalrymen).
Peter apparently gave 5 separate talks to these Roman equestrians. His talks were all faithfully recorded by his stenographer Mark. Hence the title Kata Markon -- "[the Gospel] According to Mark"!
For more, see my book on the subject.