Following is an entry for the Giant Hippopotamus, one of the creatures that appears in Skirmisher Publishing's bestselling "Men & Monsters of Ethiopia"! 

Hippopotamus, Giant

Gargantuan beast, unaligned

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Armor Class 14 (natural armor)

Hit Points 140 (10d20 + 30)

Speed 40 ft., swim 40 ft.

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STR                DEX               CON               INT                 WIS                CHA

29 (+9)            10 (+0)          22 (+6)           2 (−4)             12 (+1)            8 (−1)

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Skills Perception +4

Senses passive Perception 14

Languages —

Challenge 7 (2,900 XP)

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Reckless. At the start of its turn, the giant hippopotamus can gain advantage on all melee weapon attack rolls it makes during that turn, but attack rolls against it have advantage until the start of its next turn.

Trampling Charge. If the giant hippopotamus on land moves at least 20 feet straight toward a creature and then hits it with a melee attack on the same turn, that target must succeed on a DC 19 Strength saving throw or be knocked prone. If the target is prone, the giant hippopotamus can make one stomp attack against it as a bonus action.

Submerged Stealth. Although the giant hippopotamus is too hulking to employ stealth while on land — having no proficiency in it and suffering disadvantage on Stealth skill checks — the creature has proficiency and advantage with it in water deep enough to conceal itself (Stealth +2). 

Actions

Bite. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 18 (3d8 + 9) piercing damage.

Stomp. Melee Weapon Attack: +12 to hit, reach 10 ft., one prone creature. Hit: 21 (3d10 + 9) bludgeoning damage. 

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.