My nominee isHAMBURG b.h. 1895 (Hanover - Lady Reel).
This horse performed deeds as a 2yo even 4yos would have a hard time replicating!
He made his first start June 9, 1897, and the last September 16, making 16 starts in all. he won 12, was 2nd 3 times, 6 of his starts were in September alone!
Anyway, his individual stats are simply mindboggling:
July 5 - Great Trial Stakes - 122 lbs - w-t-w
July 10 - Double Event Stakes (section 2)* - 129 lbs - w-t-w
August(?) - Congress Hall Stakes* - 134 lbs, gave 15 lbs to runner-up
Agusut(?) - Grand Union Hotel Stakes - 129 lbs, beaten a head by Archduke (117 lbs)
August 17 - Rising Generation Stakes* - 127 lbs, beat Central Trust (122 lbs), & Julius Caesar (111 lbs)
August 21 - Electric Handicap (!) - 132 lbs, beat Handball (120 lbs), First Fruit (88 lbs)
September 1 - Flight Stakes - tried to give the 4yo Requital 5 lbs by scale. Requital beat him 2 lengths, but he was 10 lengths in front of the next horse
September 6 (he had a race between this and previous) Autumn Stakes* -- 129 lbs
September 11 - Great Eastern Handicap (this race was the most impressive) - 135 lbs (!) - stood at barrier for half an hour, went w-t-w, beating Kitefoot, and George Keene (both 111 lbs); the lightweight in the field got 40 lbs from him.
September ?? - Prospect Stakes* - 127 lbs
September 16 - Excelsior Stakes
*"allowance" race
Hamburg raced only 5 times at 3, but his last two races (he won both) were at 13f, and 18f; which rather belied his earlier rep as a "mere sprinter".
(sources: "Names In Pedigrees" by Joe Palmer, and "The History of Horse Racing in America" by William Robertson)