Following is an entry for the Hippopotamus, one of the creatures that appears in Skirmisher Publishing's bestselling "Men & Monsters of Ethiopia"! Hippopotami, which can be found in Ethiopian waters that include Lake Tana and the Blue Nile, appeared in various early editions of Dungeons & Dragons/1st Edition D&D Monster Manual, but for some reason were not picked up for the 5th Edition D&D game until game developer Michael O. Varhola adapted it for the afore-mentioned book.
Huge beast, unaligned
Armor Class 12 (natural armor)
Hit Points 52 (5d12 + 20)
Speed 30 ft., swim 30 ft.
STR DEX CON INT WIS CHA
21 (+5) 10 (+0) 18 (+4) 2 (−4) 12 (+1) 8 (−1)
Skills Perception +3
Senses passive Perception 13
Challenge 3 (700 XP)
Reckless. At the start of its turn, the 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 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 15 Strength saving throw or be knocked prone. If the target is prone, the hippopotamus can make one stomp attack against it as a bonus action.
Submerged Stealth. Although the 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).
Gore. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 18 (3d5 + 5) piercing damage.
Stomp. Melee Weapon Attack: +7 to hit, reach 5 ft., one prone creature. Hit: 21 (3d6 + 5) bludgeoning damage.