The parents of one of my best friends had a Belgian shepherd named Troushka.
They’d had many Troushkas before (and all Belgian shepherds), but this one was a tad different.
This one attacked every single “intruder” on the premises — including the mother of the house, and her kids — and he only (kind of) listened to my friend’s father — a hairy 6′6′’ tall Northman who resembled a bear more than anything else.
One day, my friend and his brother noticed that all the chickens and ducks in the farm had disappeared — only to find out that Troushka had buried them one-by-one, alive and head first. Some had lost a leg (or an eye, for that matter), but every single one was still alive when the two brothers saved them in the end.
(On more than one occasion, I was saved from Troushka myself by my friend and his brother, and without them I won’t even dare to think what would have happened.)
Troushka eventually died when he attacked a car that passed in front of the house of my friends. He first jumped over a 6′ high picket fence in order to achieve his goal, by the way.
Which brings me to your question —
“Why don’t we see mentally challenged animals ?”
Answer: We actually see them all the time. But if we choose to ignore mentally impairment in humans on a daily basis —
Then how could we even see the challenge in other animals ?
SOURCES: the Dover Street Studio, Public domain, via Wikimedia Commons.