As nouns the difference between hippopotamus and dog

is that hippopotamus is a large, semi-aquatic, herbivorous (plant-eating) African mammal (species: Hippopotamus amphibius) that spends most of the day living in water, but comes on to land at night to feed. Of all living land animals, only the rhinoceros and elephant are larger while dog is a mammal, Canis lupus familiaris, that has been domesticated for thousands of years, of highly variable appearance due to human breeding.

As an acronym DOG is

digital on-screen graphic.

As a verb dog is

to pursue with the intent to catch.