Hippopotamus

A hippopotamus on the party's side is a druid's wild shape, a savanna ranger's bond, or a bull the party rescued from poachers. Use it as a heavyweight in water. Walk speed 30, swim speed 30, and outside the river it's slow and conspicuous. In any waterway deeper than 4 feet, it's a CR 4 wall to hide behind.

In combat, use the Shove. The hippo surfaces next to an enemy boat and capsizes it. It stands in a ford and refuses to move while the party shoots from cover. It drags a downed PC from the river onto the bank. The Bite at +7 for 2d10+5 piercing is the finishing move, not the opening.

Out of combat, the hippo is a mascot that eats grass, snorts at strangers, and won't enter buildings. Use it for scenes where the party ferries cargo across a crocodile river or hides from patrol by submerging behind the bull. The druid form gives 82 HP, AC 14, and 30 feet of swim in any deep water.

When the bond ends, the hippo bellows once and disappears into the reeds. The party should feel the river get smaller without it.

A hippopotamus is a CR 4 territorial predator with 82 HP, AC 14, Strength 21, Hold Breath, Multiattack, and Bite at +7 for 2d10+5 piercing. Real hippos kill more people in Africa than lions. Run this with that weight. The hippo surfaces under a boat and capsizes it in the first round. Round two is Bites on whoever surfaces.

Use the water as the trap. The hippo has been watching from reeds with Perception +3 and swim speed 30 feet. It positions beforehand. On initiative, it Shoves the boat with Strength 21 against party Acrobatics. The boat goes over, casters spill into the river. Round two, Multiattack with Bite on the first PC to surface, dealing 2d10+5 piercing.

In the water, the party climbs swamped boats or treads water in armor. The hippo hunts one target and drags them under. Run this as a save-or-half: Athletics or Acrobatics escape brings them back to the gunwale. Failure costs a turn underwater taking bludgeoning from thrashing. The hippo has Hold Breath at 10 minutes, so it doesn't panic.

The hippo retreats once bloodied if its calf is safe, charges again if the calf is in the boat. Decide before the scene. Have a player roll Nature DC 13 to spot the calf on a sandbar before initiative. A party that sees it avoids the fight. A party that doesn't learns about hippos the hard way.