When an animal's back curves down in the middle, it is called swayback.
Swayback can make people think of old animals or animals that have worked hard.
Examples:
The old horse had a swayback.
The swayback horse was very gentle.
The swayback made it hard for the horse to carry things.
The swayback horse looked like a bridge.
The swayback on that horse was so big, it could carry a person on its back.
The swayback on the horse made it look very old.