Complex sentence: In its equine form the kelpie is able to extend the length of its back to carry many riders together into the depths of the water, a common theme in the tale of several children clambering onto the creature's back while one remains on the ground.
Split into simpler sentences: In its equine form it was able to extend the length of its back to carry many riders together into the depths of the water. It is a common theme in the tale of several children clambering onto the creature's back with one child remaining on the ground.