Walter Farley, The Black Stallion's Blood Bay Colt, in which the Black sires a son, Bonfire, on a harness-racing mare

Thanks for this entry, I obsessively read and collected the books as a child, even the one where
Alec drops acid, but I've never heard of this title! Consider me rushing off to buy it from Amazon. (How could I have missed it?! Did my purist parents keep it from me because it was about harness racing?!) *g*