That seems unlikely. Source?
Actually, I was wrong, appears there was a big spike around the war and after, I always went off there being more now than at turn of century. There are close to 10 million in US now, which was close the the turn numbers or a bit over if you don’t count wild ones then (of which there were a lot), but it peaked out in the war years and immediately after.
Regardless, buggy whip and pony spanker makers are doing well...
The horse population dropped dramatically following conversion of urban transportation from horse to motorized automobile and rail.
I think the horse population in the US peaked around 20mm by end of WWI(from memory). This site lists a peak of 25mm as of 1920: https://www.horsetackco.com/horse-blog/horse-population-in-the-usa/
Check out this article: The Great Horse Manure Crisis of 1895: http://bytesdaily.blogspot.com/2011/07/great-horse-manure-crisis-of-1894.html