The item you are thinking of might be "knacker's cart".

A long time ago, there were special carts called "knacker's carts" that were used to take away dead horses and other animals. The people who drove these carts were called "knackers" because they bought and got rid of the dead animals. You might have seen these carts in your grandparents' stories or in old movies, but they're not used anymore because we now have better ways of getting rid of dead animals.