Hippopotamus

A hippopotamus on the party's side is a druid's wild shape, a beast bond from a savanna ranger, or a bull the party rescued from poachers and that now follows their barge upriver. Treat the hippo as a heavy that lives in water. Out of the river, it is slow (walk 30, no climb) and conspicuous, but in any waterway deeper than 4 feet it is a CR 4 wall the party can hide behind.

In combat, the play is the Shove. Improvise a Bite at +7 for 2d10+5 piercing if the table needs a damage line, but the hippo's real value is mass and intimidation. Have it surface next to an enemy boat and tip it. Have it stand in the ford and refuse to move while the party shoots from cover. Have it drag a downed PC out of the river and onto the bank because the player wanted that to happen.

Out of combat, the hippo is a mascot and a transport problem. It eats prodigious amounts of grass, snorts at strangers, and refuses to enter buildings. Use it for the kind of scene where the party needs to ferry their gear across a crocodile river or hide from a flying patrol by submerging behind the bull's bulk. The druid form gives the player a clear toolkit: 82 HP, AC 14, and a 30-foot swim speed in any water deep enough to count.

When the bond ends (the druid drops concentration, the rescued bull goes back to its herd), have the hippo bellow once and disappear into the reeds. The party should feel the river get smaller without it.

A hippopotamus is a CR 4 territorial encounter that exists to remind a low-tier party that nature is not their friend. Real hippos kill more people in Africa than lions. Bring that energy. The stat block has no listed actions, but with Strength 21, 82 HP, and a 30-foot walking and swimming speed, this is a charging mass that opens at the riverbank and ends when the boat capsizes. Improvise a Bite at +7 for 2d10+5 piercing, which is faithful to its size and Strength and gives you something to roll.

The hippo opens by surfacing under the boat. It has been watching the party from the reeds with Perception +3, and its swim 30 means it gets where it wants to go in the water without effort. The first round is a Shove (Athletics is not listed but Strength 21 against most party Acrobatics is a roll the hippo wins) to capsize, prone, dump the casters into the river. Round two is a Bite on whoever surfaces first.

In the water, the party has problems. They are climbing back into a swamped boat or treading water in armor (Strength check or sink). The hippo is at home. It picks one PC and drags them under. Run this as a save-or-half rather than a meat grinder: an Athletics or Acrobatics escape gets the PC back to the gunwale, a failure costs them a turn underwater taking 1d6 bludgeoning from the thrashing.

The hippo retreats once Bloodied if its calf is safe, charges again if the calf is in the boat. Decide before the scene starts which it is, and let the party feel the difference. Have one of them roll Nature DC 13 to spot the calf on a sandbar before initiative. The party that sees it can avoid the fight; the party that does not learns about hippos the hard way.