Snails and turtles are two that do this.
Monkeys and Gorillas mainly. Certain marsupials continue to carry their young on their back for several months, once the joeys have emerged permanently from the pouch. These are mainly the arboreal...
Some marsupials carry their young on their back once they are old enough to leave the pouch. Possums and koalas are examples of this.
A sloth is one.
Camel