Cockatrice

 The still valid, but somewhat underwhelming way to utilize a cockatrice, is as the poor man's basilisk, when the PCs are low level. The way to make a creature with a single special ability that stacks with itself memorable, though, is to wait for the PCs to level up, then throw heaps and heaps of these annoying creatures at them. Do note: Some of the entries below will substitute the bestiary's origin of the cockatrice for european folk tales about it.

A seer was fired from the King's court for treachery. What the man did was suggest that a rooster laying eggs in the king's royal garden is a bad omen, and that said eggs should be destroyed or at least removed. Unfortunately, the king found the occurrence to be very interesting, and has invited guests from several neighbouring lands to marvel at the eggs.
The seer, still loyal to the foolish monarch, recruits some of the visiting dignitaries entourage to destroy the eggs - unfortunately, as they arrive at the gardens in the dead of nights, several eggs appear to have already hatched, and statues apparently depicting terrified work staff and at least one pair of lovers are scattered among the rare plants.

A goblin alchemist working in the sewers of Kepesh has made a radical discovery. By submerging cockatrice eggs in specifically curated refuse from great beasts from the surrounding plains, he has managed to breed several mutated variations of the base cockatrice, ranging from those with breath weapons, to large, amphibious breeds with alligator-like jaws who draw their slowed prey into the muck to drown them. Unfortunately, as accomplished as the goblin is at breeding the beasts, he hasn't yet found efficient ways to control them - so now the already perilous undercity has grown ever more dangerous.

The harshest of the many forms of duels in the nation-state of Ciri'Tarya is the so called slowpoke duel. It is an often deadly endeavour, where participants are given a rapier, no armour, and a dose of high-proof cockatrice venom each. It is only used for the gravest of offenses, as a single good hit can often end the entire affair.
Unfortunately, the party's bard flirted with the wrong noble's fiancee, and a challenge has been issued. By law of the land, such a challenge can not be refused - but the bard's blades are as dull as his words are sharp.