|Overloaded donkey cart - Unfortunately a common sight of animal mistreatment in South Asian countries|
Mistreatment of animals in Islam is a sin. It’s as simple as that. No questions asked!
It is related from Jabir (in Muslim) that the Messenger of Allah (SAWT), once saw a donkey who had been branded on its face and he said, "May Allah curse the one who branded it."
This is how bad hurting an animal is considered. They are innocent creatures of Allah and no human has the right to take their life (unless for the purpose of religious sacrifice or for the need of food) or hurt them in anyway. Even for animals we use for carrying burden, like horses or donkeys, we have been asked to show mercy on.
So you see, even our religion doesn't allow any mistreatment towards animals. What then makes us think that we can pelt stones at a street dog or run over a cat on the road or overload a donkey with burden?
Our religion teaches us kindness towards all creatures, specifically those who cannot speak for themselves. We must become their voice and the least we can do for them is show kindness and teach the same to our children.