I think most people would be shocked if they only knew what animals were flying with them or occupied their space last. They might even be more shocked if they knew what they had left behind.
One of the biggest problems caused by animals on planes is corrosion. For example I remember British Caledonian used to very often fly planes with passengers in one direction and then use the same plane to ferry race horses and even elephants the other way. In itself, not a problem as long as the interior was put right before and after but there became one issue that never seemed to disappear completely, and that was poo and primarily pee.
Sometimes these animals used to pee so much that when the aircraft tilted upwards a veritable tidal wave of faeces ridden pee rushed to the rear of the aircraft engulfing the ankles of any unwary staff that had their seats there. I know as I have a ruined pair of trainers to prove it! It used to then get crazier if the passengers were elephants as they, like us, get bored, but when they do their heads sway from side to side. You get more than one doing this in sequence and it causes chaos with the autopilot.
Anyway, back to the pee. This pee has to go somewhere and, unless the cabin is sealed like a Tupperware box that somewhere ends up being under the floor and amongst the aircraft structure itself. This stuff is full of ammonia and, when sloshing around on sheets of alloy, nuts, bolts and wires nature starts to follow its course resulting in various levels of corrosive threat to the aircraft infrastructure. These problems were always found during the major maintenance ‘D’ check which such planes went through but who would have thought that a Jumbo could have done so much damage to….well a Jumbo!