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 with 5 HP and AC 12 means 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. Run it that way.

The stat block has no actions listed, so improvise a Bite at +3 for 1d6+1 piercing if the table insists on a roll. The real toolkit is sensory: Perception +5, passive 15, Darkvision 60. 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. 5 HP dies to one javelin, and the player who lost their dog will remember it for the rest of the campaign. 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. Do not kill the dog unless the player is ready for that scene.

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, and it costs you nothing to make that real.

A mastiff as adversary is a guard dog the party has to handle without killing, which is the more interesting version of the encounter. 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 (5 HP, AC 12, no real attack), 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.

Run a mastiff or pack of 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 fail, barks (Shriek-style alert 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 (Sleep, Animal Friendship), or with an inside contact who knows the kennel. A Druid speaking via Speak with Animals can negotiate.

If a fight does break out, run the pack as a single morale unit. Three mastiffs at +3 to bite for 1d6+1 each 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.