Appears they aren't COMPLETELY vegetarian.
Like almost any herbivore, they will consume many other plants if presented with them, but their diet in nature consists almost entirely of grass, with only minimal consumption of aquatic plants.
[40] Hippos have (rarely) been filmed eating
carrion, usually close to the water. There are other reports of meat-eating, and even
cannibalism and
predation.
[41] The stomach anatomy of a hippo is not suited to carnivory, and meat-eating is likely caused by aberrant behavior or nutritional stress.
[5]
The diet of hippos consists mostly of terrestrial grasses,