Paul Revere was a figure from Earth's history. A horse-ride he performed was somehow essential to America, previously a British colony, gaining its independence. A Professor imploring two other scientists who were experimenting with time travel to be less reckless, lest they alter the course of history, once used the example that the iron ball they sent back in time as an experiment, arriving at the wrong place and time, could have spooked Revere's horse and interrupted his ride, altering Earth's history in major ways. (COMIC: Dr. Who's Time Tales)
A hypothetical scene of Paul Revere getting thrown off his horse during his famous ride. (COMIC: Dr. Who's Time Tales)