I am not familiar with horse racing at all. Just watch the three big races for fun. There is probably a good reason that Secretariat was the only animal named in the top fifty athletes of all time. So I'm gonna go with that.
IMO, with all due respect to War Admiral, Seabiscuit, and Man O'War, it comes down to Secretariat and Phar Lap.
It would have to be Secretariat until he attempted a comeback against Mike Tyson in that dreaded fight in international waters. Wait I don't care about horse racing.
Don't forget that ESPN is an American network. Secretariat won 16 of 21 races. Pharlap won 37 of 51, including a stunning 36 of his 1st 41 before encountering minor health issues. Came over to North America for the richest race of the day ( Agua Caliente Handicap) and won going away. Dies soon after under mysterious circumstances most believe to be the work of gangsters.
Does anyone know the track records at the triple crown races? I believe Secretariat holds one - I think this would tell us a lot.
Tha 'Biscuit' was the real deal - came back from a torn knee ligament raced until he was 7 took care of War Ad in a head to head. What more can you ask? I think he's number 2 behind Secretariat.