Most animals slaughtered for human consumption are inspected only for a few seconds after they've been dismantled.

I worked in a massive industrial-scale slaughterhouse for a while. The place met all the health regulations of New Zealand, the UK and the US, since it was mostly killing for export. They killed a lamb every ten seconds. The inspectors could look at the carcase and the guts together, because the conveyor moved them in parallel, and if there were any obvious signs of disease (usually "sheep measles") the animal would be condemned and taken off the line. But with so many lambs going through the system, I doubt many of them got much more checking than whether they could stand and walk.

With chickens the numbers are even more extreme. My guess is the only inspection they get is at slaughter by automated image recognition systems, if that.

A horse is a big and expensive enough package of meat that its carcass probably gets looked at with a bit more care.