I'm not totally surprised with this picture because Einstein was the first to propose the theory that a Pegasus could achieve, what he called, the Sonic Rainboom. He published his theory in 1905 along with his other works but it was mostly overlooked and few copies were printed because it was believe to be a joke. Those few that got into the hands of Physicists criticized his theory for lack of understanding the amount of G forces a Pegasus could handle. The theory couldn't be tested because of lack of funds and interest in the project. None of the papers that speak in detail about it exist but it is mentioned in a letter Einstein never sent where he explains what it looked like. The person who the letter was for has a black spot over where the name would be.
It wasn't until recently that people gained interest in his theory. The reason for this is tranced toward an incidented that occured during the Best Young Flyers competition where a Pegasus named Rainbow Dash was able to produce what is called the Sonic Rainboom. There is no mainstream theory that explains why a sonicboom happens just that it does.
(For some reason I thought that it would pretty funny to write this. Why? I don't really know!)