But how would we know if an animal had mental illness? Maybe it happens and we just don't realize it. I had a cat that used to get pretty wacky at times, in fact, I've had several cats like that.
Interesting point..
Many male primates ( chimps gorillas, etc..) have onsets of extremely violent behaviour as they get older..
This has been explained as "typical" male dominance behaviour, but sometimes results in deaths of members of the "tribe"..
Possibly the violent behaviour has it's roots in some sort of schizophrenia or other psychotic ailment..
I've had several "insane" cats. Had to put the last one to sleep in June following a brain tumor that followed a head injury. Not that he hadn't been "insane" prior to that whole chain of events. He was a feral kitten from a nuclear power plant and adopted me. He scanned clean - no outward contamination, but that doesn't mean something wasn't wrong on a genetic level.