Yes, you're right. Secretariat was the first horse I remember watching - I was 10(ish) at the time. I did some quick research. Secretariat still holds the track records at both Churchill Downs and at Belmont. And, his Belmont win is the fastest time recorded on a dirt track of that length ever.
Sports Illustrated did a feature on Secretariat several years after his death. It was the most moving sports biography I'd ever read to that time and since.
Secretariat’s Preakness time has long been under dispute; in any case it’s likely his actual time was no higer than 1:53 2/5. However, given his times in the Derby (very fast) and the Belmont (I doubt that one will EVER be broken) I’m more than inclined to believe he owns the record at the Preakness. I’m sorry, the clocks just have to be wrong.
Another thing - Secretariat owns those records because he got faster as the race went along, which is almost unheard of. It would be like a marathon runner putting up better times in the last 13.1 miles than in the first 13.1. Racers of any species typically are relatively consistent, tapering off only slightly. Secretariat in the Belmont especially just ran like they were all standing still and ended up winning by 31 lengths. Youtube has a video from the day, and you can see how much the camera had to zoom out to try to get the other horses in the frame. Sham, a great horse in his own right, quit coming around the third turn.