Owl

The owl is the familiar that wins more sessions than any combat ability. CR 0, 1 HP, fly 60 feet, passive Perception 15, Stealth +5, Darkvision 120 feet. It has Talons at +3 doing 1 slashing damage and Flyby for escaping without opportunity attacks. Most parties acquire one through Find Familiar, a druid's wild shape, or a ranger's animal companion variant.

In play, the owl is reconnaissance. Send it ahead to peer into the next chamber, perch in the rafters of the tavern the party is casing, or follow the suspect through the night market. Flyby prevents opportunity attacks as the owl banks away from a swatting hand. Pair the owl with the Help action from Find Familiar: the wizard's familiar dive-bombs the goblin's face and the rogue's next attack lands with advantage.

The fragility is the design. 1 HP means any area effect the party catches itself in vaporizes the owl, and a single arrow ends it. Players who care about their familiar play more cautiously, which is good for pacing. If the owl dies as a Find Familiar, the wizard re-summons in an hour with a 10 gp pouch. If it was bonded, the loss should land.

Out of combat, use the owl to deliver short messages, scout high terrain, and harass NPCs the party wants to test. Have the owl arrive on the windowsill in the third scene and the players will know who sent it before you say anything.

A hostile owl is a stretch. The framings that work: swarms of owls under a druid's command, a single owl polymorphed into an agent, an owl carrying a cursed message that explodes when read.

If you do run owls as enemies, run them in numbers. Six owls swooping through a forest, all controlled by an enemy druid, function as information-denial rather than damage. They harass casters with constant Flyby passes, draw opportunity attacks that cost the party reactions, and report back to their handler. The actual fight is with whoever sent the owls.

A single-owl assassination angle works in a city game. An owl carries a thumbnail-sized vial of contact poison in its talons, drops it onto the target's wineglass from a rooftop, and is gone before anyone looks up. Passive Perception 15 and Stealth +5 mean most NPC guards never notice. The party investigating a poisoning has to figure out the bird before the poisoner, which is a fun mystery beat.

Have the owl land on the body after the deed, watching.