The early award for most pedantic statement goes to GalileoS. Come collect your prize which this week is a solid slap to the back of your head and an autographed copy of Neil deGrasse Tyson's latest book "Nobody Asked but I'm Going to Point it out Anyway".
This is implied from the question. In any case, Mansa Musa was just one of many rich people in history and far from the richest. Do you think the emperors of ancient Rome or China didn't have the equivalent of a billion dollars?
There was no dollar exchange rate in the 14th century, so Mansa Musa's wealth just wasn't worth $1 billion, any more than he could have bought a flatscreen TV with it. If you interpret the question to mean "worth the equivalent of $1 billion at today's value", then Rockefeller wouldn't even be the first American - Vanderbilt would easily qualify (and there might have been someone earlier for all I know).
Secretariat's running of the '73 Belmont Stakes is possibly the most electrifying sporting event I've ever seen. He ran each quarter faster than the previous one, which is impossible. He might be the most universally beloved animal that's ever lived.
Shoutout to the Beastie Boys (unironically my favorite band with these two songs being some of my favorites) for helping me get two of these questions.
Shadrach: “I’m madder than Mad’s Alfred E. Neuman”
Root Down: “MCA grab the mic and Ma Bell will connect you”
This is implied from the question. In any case, Mansa Musa was just one of many rich people in history and far from the richest. Do you think the emperors of ancient Rome or China didn't have the equivalent of a billion dollars?
Shadrach: “I’m madder than Mad’s Alfred E. Neuman”
Root Down: “MCA grab the mic and Ma Bell will connect you”