Owl

The owl is the familiar that wins more sessions than any combat ability ever will. CR 0, 1 HP, Fly 60 ft., passive Perception 15, Stealth +5, Darkvision 120 ft. The SRD entry has no actions, so this is not a damage dealer. It is a flying scout with the senses of a small drone and the deniability of a wild animal. Most parties acquire one through Find Familiar, a druid's wild shape, or a ranger's animal companion variant.

In play, the owl's role 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 (in stat blocks that include it) prevents opportunity attacks, but even without that the 60 ft. fly speed plus 120 ft. darkvision means the owl outruns anything that wants to swat it. Pair the owl with the Help action when it is delivered via Find Familiar: the wizard's familiar dive-bombs the goblin's face and the rogue's next attack lands with advantage. That single trick is the entire combat case for the owl.

The fragility is the design. 1 HP means any AOE 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 and was a familiar, the wizard re-summons in an hour with a 10 gp pouch; if it was a bonded animal, the loss should land. Don't kill the owl casually. Kill it once, with a name, and the table will remember.

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

A hostile owl is a stretch. The framings that work: a swarm-equivalent of owls under a druid's command, a single owl that has been replaced by a polymorphed agent, or an owl carrying a cursed message that explodes when read. The owl itself is not a credible combatant against any party with hands.

If you do run owls as enemies, run them in numbers. Six owls swooping through a forest scene, all controlled by an enemy druid, function as an information-denial encounter rather than a damage one. They harass the party's casters with constant flybys, draw opportunity attacks (and die from them, which costs the party reactions they wanted for something else), and report back to their handler. The actual fight is with whoever sent the owls. Treat the owls as a system the party must dismantle to reach the real enemy.

The 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 plus Stealth +5 means most NPC guards never notice. The party investigating a poisoning has to figure out the bird before they figure out the poisoner, which is a fun mystery beat.

Have the owl land on the body after the deed, watching. The image stays with the table.