Crocodile

A crocodile on the party's side is a druid's wild shape, a lizardfolk shaman's pact-bound guardian, or the temple guardian of a swamp god the party has paid tribute to. The framing is rare but it works at the table because crocodiles are simple enough to run as a single instinct: bite the thing in the water.

In combat the crocodile is a debuff with teeth. Drop it in a river or pond near the encounter and let it surge out at any enemy who tries to cross or flee. Its 30 ft. swim speed beats almost any humanoid in water, and the grapple element of a crocodile bite shuts down a fleeing enemy more reliably than most low-tier abilities. The 13 HP means it dies fast if focused, so don't expect it to tank; expect it to deny terrain.

Run the crocodile by table fiat, not by negotiation. The druid says "the crocodile attacks the bandit who's swimming away." The crocodile does that. It does not flank, ambush in coordination, or break off when the player wants it to break off. Once it has prey, it holds. If the party's plan requires the crocodile to release a target on cue, that plan needs a backup.

Name the crocodile if the players bond with it. They will. They always do.

A crocodile is an environmental hazard with a stat block. CR 1/2, AC 12, 13 HP, 30 ft. swim speed and 20 ft. on land. The encounter is the river, not the croc. Players who walk past a slow-moving brown stretch of water without rolling Perception are the target audience for this monster.

Set the ambush at the water's edge. Crocodiles have +2 Stealth and a Dex of 10, so they're not invisible, but they are flat-bottomed logs in murky water until something steps within reach. The fight resolves in two or three rounds because 13 HP folds to a single martial hit, but the surprise round is where the damage happens. A grappled or dragged PC who fails a Strength check goes underwater, and now the rest of the party is rolling Athletics or Acrobatics against the current while their friend takes drowning damage. That's the whole encounter shape: a brief panic, a save-or-suffer, and a dead reptile.

Use them in groups of two or three for any party past tier 1. A single croc is a speed bump. Three crocs in a fording scene, with one dragging the heaviest-armored PC into deep water while the other two snap at the rescuers, is a memorable five minutes. They have no traits, no ranged attacks, and no judgment beyond "that smells like food," so don't try to run them as tactical opponents. They just hold what they grab.

Describe the bite as a clamp, not a chew. The horror is the lock, not the wound. A crocodile that won't let go is scarier than one that bites twice.