Ziggurat is correct. The flight ABD4592 was a cargo B747 (registration TF-AMM)

I can’t find cabin pictures but those cargo are loaded with huge pallets, for live animals it’s the same type but adapted, kind of a big metal box but equipped with a door, probably a window and some food/water. At least one human is supervising the cabin but it could be more according to regulations (if it’s the same as for pax) or simply how much the client is willing to pay ?

So the pilots knew it via the aircraft intercom. There’s no chance they would have heard the horse in a 30 years old 747 nor feel any change of center of gravity, specifically on autopilot at cruise level.

Fun fact, the company is bragging about being the Iron Maiden or Olympic flame carrier