Basilisk

A friendly basilisk is a wizard's idea. Specifically, a wizard who needed a low-maintenance guard for a vault and decided that a slow lizard with a gaze attack was easier to feed than a golem. The basilisk doesn't befriend the party, it tolerates them because the wizard said so, and as long as the party stays on the right side of the wards or the whistle or the iron collar, it leaves them alone.

Use it as a passive obstacle that the party can choose to mobilize. If the wizard is dead or distant, the party can release the basilisk into the vault that the villains are currently looting. Petrifying Gaze does not distinguish friend from foe, so the players have to decide how confident they are in their own Constitution saves before they kick the door. That's a real decision, not a reward.

In actual cooperative combat, the basilisk holds a doorway. The 30-foot cone covers a corridor cleanly, the bite punishes anything that closes, and the AC 15 plus 52 HP means it can absorb a round or two of attention while the party flanks. Tell the party to pick a side and stay on it. Anyone who walks in front of the basilisk's face is rolling a DC 12 Con save, friend or foe, and the basilisk is not going to remember names.

The exit is simple. Once the immediate threat is dealt with, the basilisk goes back to its corner. It does not follow. It does not learn. Treat it as a reusable trap, not a companion.

A basilisk is a puzzle wearing a stat block. The bite is a respectable +5 to hit for 17 average damage, but the actual encounter is the gaze, and the actual encounter is whether the party figures out the gaze before someone is a statue. Sell the puzzle. Statues in the lair, half-petrified rangers reaching for arrows, a hunter's cairn with the inscription "do not look back". The party should arrive already worried.

Petrifying Gaze is a 30-foot Cone, DC 12 Constitution save, on a Recharge 4 to 6. First failure is Restrained with a save at end of next turn, second failure is Petrified outright. The DC is low enough that most level 3 to 5 PCs save, but two consecutive bad rolls can take a wizard out of the campaign permanently. That's the threat. Make sure the players know there's a way out before you start rolling petrification saves: averted eyes (attack at disadvantage, basilisk has advantage), mirrors (basilisk targets itself if it sees its reflection), or fighting from outside the cone.

Mechanically the basilisk is slow. 20 ft. walk, AC 15, 52 HP, no ranged option. The gaze is a Bonus Action, so its turn is bite plus gaze plus shuffle. If the party splits to flank from outside the 30-foot cone, the basilisk has to choose which one to face, and the other gets free attacks at advantage. Reward this. The encounter is fundamentally about geometry.

Retreat is unlikely. This is an Int 2 monstrosity in its own lair, and it has nowhere better to go. If the party brings mirrors and forces the basilisk's own save, it stops eating things and starts hiding. Don't grant a long rest in its lair, but do let the party leave alive after a clever scene, scarred friend in tow.

Drop one petrified adventurer in the entrance with a still-readable journal. The party will not look at the basilisk after that.