Animals are classified as heterotrophs because they use other organisms are their carbon source.
The purpose of a heterotroph is to obtain carbon from other organisms. Animals do this all the time. An example of animals being heterotrophic would be how a horse grazes on grass and hay. Because grass is a producer and emits carbon, a horse would be considered a heterotroph. This same comparison can be made with practically all animals, because they gain carbon from other organisms.