This is an example of assimilation.
According to Piaget, assimilation is a stage in a child's development process during which it introduces new information to his already existing knowledge. So, Joey knows that a horse has 4 legs, it is big, has a mane, and hoofs. Zebra is quite similar to a horse (except for the color and some other characteristics), so a child can confuse the two if it has never seen a zebra before.
