Spider

A friendly spider is a familiar (Find Familiar produces a tiny beast and the spider is right there on the list), a druid's wild shape into a Tiny beast, or a witch's pet that the party has had to coexist with since the first session. The animal has 1 HP and an Intelligence of 1, so the framing is "tool that walks", not "companion".

The use case is reconnaissance. Walk and climb 20 ft. each means the spider can scale a wall the rogue can't reach, slip through a one-inch gap under a door, and report back through the familiar bond if it's a familiar. Stealth +4 with Darkvision 30 ft. means it sees in pitch dark and rolls well to not be noticed, which is the whole job. Use it for scouting the next room, mapping the corridor, identifying the patrol pattern. Do not put it in combat. AC 12 and 1 HP means a sneeze ends it, and if it's a familiar, the caster has to spend the spell slot again to get a new one.

The texture is the squeamishness. Half the table loves having a spider familiar and half the table hates it. Play the comedy hard: the spider on the bard's shoulder during a noble's audience, the spider that the innkeeper finds in the soup, the spider that the paladin steps on by accident and has to apologize to the wizard for.

When the spider does scout something the party needed to know, narrate it in one clean sentence and move on. The spider earns its keep in two-line check-ins, not in extended scenes.

A spider is a CR 0 prop with 1 HP. There is no actions block, no bite, no web, nothing. Treat it as scenery with a stat line. The point of the spider in the bestiary is that the room had spiders in it, the players brushed them off, and somebody made a Stealth check at +4 because there were spiders to provide cover. The honest fight is no fight at all.

The way to use the spider as a threat is in volume and atmosphere. A single spider on a doorframe sets the tone for an abandoned crypt. A dozen spiders skittering across the wizard's bedroll while she tries to sleep is a Constitution save against losing a spell slot to an interrupted rest. A nest of them dropping from the ceiling onto the rogue who just disturbed the wrong thread is a Dex save against being startled into the trap she was about to disarm. None of this requires the spider to roll an attack. The spider is a trigger, not a combatant.

If a player insists on initiative, run it cleanly. Walk and climb 20 ft. each, AC 12, Stealth +4, Darkvision 30 ft., 1 HP. Any hit drops it. The spider does not bite back in any rule the SRD provides. If the player wants a bite, the GM rules an improvised attack at low damage, and even a hit isn't going to register against a level 1 PC. Move past the round and back into the fiction.

The strongest play with the spider is restraint. The cave gets webs, the players get Stealth advantage from the cover, the cleric finds a egg sac that hatches in his backpack three days later. The spider does its work as flavor and the table remembers the cave.