Venomous Snake

A friendly venomous snake is a familiar, a druid's wild shape into a Tiny beast, or a serpent-cult cleric's holy companion. The snake has 5 HP and Intelligence 1, so the relationship is custodial: the handler keeps it alive, the snake does what its instincts allow, nothing more.

The use case is intimidation and reconnaissance. The 30 ft. swim is rare at low levels, so a familiar that can scout a flooded chamber or follow a riverbank without surfacing earns its keep fast. Walk speed 30 ft. and Stealth from the small size means it slips into rooms the party can't approach openly. Blindsight 10 ft. is a short bubble but useful in pitch dark for the last few feet of any approach. Do not put the snake in a fight. AC 12 and 5 HP means it dies to one solid hit, and if it's a familiar, the caster has lost a spell slot.

The social texture is the point. A snake draped on the bard's shoulder ends most market negotiations early. A snake on the table during an interrogation says more than any Intimidation check. The party that travels with a venomous snake is the party that does not need to convince the village that the strangers are dangerous. They are visibly dangerous. The downside is every village watch that spots the snake will be at the door by morning.

Have the handler whisper-feed the snake at the table. Players remember the small ritual more than the stat block.

A venomous snake is a hazard wearing a stat block. CR 1/8, 5 HP, AC 12, Tiny, no actions block in the SRD. The danger isn't the bite, because the bite isn't even on the sheet here, it's the situation: the snake is in the bedroll, in the boot, coiled on the branch above the path the ranger is about to walk under. Run it as a triggered hazard, not as a combat encounter.

The setup is the encounter. A surprise round where the cleric reaches into a saddlebag and the snake strikes is more memorable than a six-round fight. Use the party's Passive Perception to determine if anyone notices the rustle in the grass before it triggers. If they fail the passive, the snake gets a free strike on whoever is closest, which the GM rules as an improvised attack with poison damage suitable for the table. Five HP and AC 12 means the snake dies the moment combat actually starts, so the question is whether anybody got bit before the snake was found.

The 30 ft. swim and 30 ft. walk make this the right monster for water encounters too. A river crossing where the rogue feels something against her leg, a swamp expedition where the wizard finds one wrapped around a staff on the bank. Blindsight 10 ft. means the snake reacts to whatever crosses into that bubble regardless of light, so a torch doesn't help.

If you want a real fight, use a swarm rule (run a bunch as one creature, with a swarm-style attack) or use the snake to set up a follow-on threat: the druid is poisoned, and the actual encounter is the gnoll patrol that arrives while the cleric is mid-Lesser Restoration. Make the snake the inciting incident.