True, but the horses' population started (slightly) rising again when they went from economic tools to recreational tools for humans. What will happen to humans?

The horse population was being boosted beyond normal numbers by human intervention. When humans stopped breeding them the numbers dropped.

At least currently humans do not need AI to reproduce.

There were approximately zero horses in the wild, so it was all about what humans found useful.

Pray it’s still humans who ask these kinds of questions about AI, not the other way around.

Did the population of work/service dogs decline? Horses were already a form of automation over human labor.

Bullocks.

That's what Sandy over the road (born 1932, died last year), used to hitch up every morning at 4am, when he was ten, to sled a tank of water back to the farm from the local spring.