Mastiff

A mastiff is the dog at the gate, the warhound chained in the courtyard, the loyal beast that travels with a paladin or a city guard. CR 1/8, 5 HP, AC 12. The mastiff is not a combat asset, it is a character. Players who acquire a mastiff are buying a friend, a tracker, and a moral compass that growls when the rogue lies.

The real toolkit is sensory: Perception +5 and passive 15, Darkvision 60 feet. The mastiff is the party's tripwire. It hears the assassin on the roof tiles before the bard does. It sniffs the cup of wine and refuses to drink it. It will not enter the room with the doppelganger because the doppelganger smells wrong. Use the mastiff to telegraph problems the party would otherwise miss.

In combat, keep the mastiff out of the line. Its Bite is +3 doing 1d6+1, which knocks a Medium or smaller target Prone, but 5 HP dies to one javelin. Have it pin a fleeing thief, drag a downed PC by the collar to safety, or hold the door against a single goblin while the party regroups. If a fight is going to be lethal, the mastiff bolts and finds the party afterwards, limping and proud.

Name the mastiff before the player does. Give it one verb: always growls at the warlock's pact weapon, always sleeps on the cleric's boots, always finds the bone the bard hid. The mastiff is the party's emotional anchor in a campaign of murder.

A mastiff as adversary is a guard dog the party has to handle without killing, which is the more interesting version. A noble's hound, a kennel of war dogs at a fortress gate, a constable's tracker that has caught the rogue's scent. Killing it is trivial, but killing the lord's prize hunting hound is a felony in most kingdoms and a personal grudge from the lord forever. Make the party feel that weight.

Run mastiffs as a Stealth and Animal Handling problem. Passive Perception 15 and Darkvision 60 means sneaking past a sleeping pack is a real DC 16 to 18 Stealth check. The lead dog wakes on a failure, barks with a shriek audible across the courtyard, and the encounter shifts from infiltration to a chase. The party can quiet the dogs with a thrown haunch (DC 12 Animal Handling), with magic, or with an inside contact who knows the kennel.

If a fight breaks out, run the pack as a single morale unit. Three mastiffs at +3 to bite for 1d6+1 each doing Prone-knock is real damage on a CR 1 party, but the pack scatters when one dies. The point is the noise and the moment the party has to choose between the dogs at their ankles and the man with the crossbow on the wall.

Have the lead dog wear a collar with the lord's crest. The party that hurts the dogs has just identified themselves to the man hunting them. Let them see the collar before they swing.