Monsters of this sort appear to be especially huge versions of normal hippopotami and can be found in the very largest bodies of fresh water in Ethiopia, particularly Lake Tana, headwaters of the Blue Nile.
A very large older giant hippopotamus can sometimes become extremely sedentary, not needing to eat more than whatever drifts into its slack jaws, and its back might actually have light vegetation sprout on it or even become a home for smaller creatures. Such a semi-dormant monstrous hippopotamus might be more than 30 feet across and easily mistaken for a tiny island (and visitors might not be disabused of this impression if the creature does not suffer any damage or become agitated). Legend tells of powerful rulers who have had small palaces built upon such creatures or used them as warbeasts equipped with fortress-like howdahs.
Like regular hippopotami, the giant variety are adept swimmers and can also travel along the bottom of lakebeds, being able to remain submerged for as much as an hour at a time. These massive herbivores are, fortunately, not generally as irritable, aggressive, or territorial as their smaller cousins — but, if antagonized, can be even more dangerous and destructive, inflicting terrible damage with their phenomenally powerful jaws. And, while they do not generally emerge from the water under passing vessel unprovoked, they do have the ability to capsize a watercraft up to the size of a small galley or merchant ship.
If two giant hippopotami are encountered one will be a female and, in that case, there is a 20% chance they will be accompanied by a calf that has statistics equal to those of a normal hippopotamus. Both parents will be very protective of their offspring.