Any organism that cannot provide its own sustenance is known as a heterotroph, and these come in every size from the large blue whale all the way down to certain types of tiny protists. All heterotrophs require sustenance from organic material that comes from other organisms, whether dead or living.
Every organism in the animal kingdom is a heterotroph, feeding on something lower down the food chain. For example, blue whales eat smaller fish, crustaceans and plankton, among other prey. Lions have a variety of prey in the wild. Some animals, such as cows and horses, rely on herbivorous diets to receive their nutrients.
Even coral is heterotrophic in nature, gaining its nutrition from the tiny algae, which are known as zooxanthellae, that live in the tissues of the coral. Protists that absorb organic material from the animal and plant tissue around them are also heterotrophs, using their own digestive enzymes to convert that material into useful nutrients.
Organisms that do not require external forms of nutrition are called autotrophs. Using light or chemical reactions, they can build the necessary carbon compounds for energy without any outside assistance. Most autotrophs are either microscopic in size or come from the plant kingdom.