No, a hippo is not an amphibian; it is classified as a mammal. Hippos belong to the family Hippopotamidae and are large, mostly herbivorous mammals found in sub-Saharan Africa. While they do spend a lot of time in the water, which can sometimes lead to confusion about their classification, they are primarily land animals.
Amphibians, such as frogs and salamanders, are cold-blooded and usually undergo a metamorphosis from a juvenile aquatic stage to an adult terrestrial stage. Hippos, on the other hand, give birth to live young and have lungs for breathing air, similar to other mammals. They are adapted to a semi-aquatic lifestyle but are not amphibians.