The myth isn't even true for harem breeders, even though a stallion, say, is designed to couple several times a day with a much shorter refractory period.

A typical feral stallion, has ten to twelve mares in "his" harem (it's really the other way around, horses lean matriarchal). Of those, one or two are probably stealth bred by other males. Dominant herd stallions don't typically get to stealth breed.

So a feral stallion typically breeds about 8 or 9 mares a year.

(A domestic stallion may indeed breed over 100, they're certainly physiologically capable, and in less than 100 days, but that's with humans lining him up for them).