Horses were never “employed”.

They did a job and got paid for it (in food and housing). Sounds like employment to me. You can call it slavery if you prefer. But that doesn’t change the fact that there were jobs that used to be done by a horse, that is done by a machine now. Meanwhile the resulting increase in productivity and market growth didn’t create new jobs for horses, they were simply no longer needed in the job market. Machines where used for all the new jobs that appeared right from the start. The horsepower the horse provided could be provided easier and cheaper by a machine.

What jobs are left for the human once AI can replace their brainpower? Blue collar jobs might be safe for a while longer until robotics catches up. But a job that is mostly shifting information around, be it spreadsheets, phone calls or art, all of that is slowly getting into reach of AI.