This hideous avian creature has the body of an emaciated rooster, the wings of a bat, and a long, scaly tail.
Source: AD&D 2E: Monstrous Manual. Original Source Dungeons & Dragons Set(1974).
The Cockatrice is a hybrid of a lizard, cock, and bat. It's known for it's ability turn flesh to stone. The Cockatrice main habitat is the wilderness where it hunts insects, small lizards, and the like. It does not use it's petrification ability when hunting for food. It only uses this ability when its lair has been threatened.
The Cockatrice has a nasty temperament. The female is the rarest of the genders and tend to be the most dominant. Males will fight for the privilege of joining their female counterpart's harem. A cave will usually house a flock of Cockatrices. Inside the cave the Cockatrices build nests and decorate them with shining objects such as coins or gems.
Combat: Cockatrices are small creatures no bigger then a large turkey with the ability to fly. They are worth 650 experience points. They have a weak beak attack that does only 1d3 points of damage. The defining aspect of that beak attack is the ability to petrify their victims. Their petrification attack will only work on those wearing fabric based armor like leather or no armor. It will not work through metal armor.
The Cockatrices know their limitations. A flock of Cockatrices will gather around the least armored individual to delivery their beak attack.
-Petrification: The Cockatrice try to attack expose flesh and those wearing only fabrics. If a large area of flesh is exposed it is assumed they attack that area. Those attacked by the Cockatrice must save vs petrification or be turned to stone. The level of AC will reduces the chance of penetration by 10% per AC value. So an AC 7 opponent has a 70% chance of the beak penetrating to the skin.
Cockatrice feathers are sought out by certain wizards for their quills which can be used to scribe certain scrolls. Also the unhatched eggs are prized for those seeking to raise one from birth. Both of those details open a couple of adventure plots. Having a wealthy merchant hire the player characters to sneak into a Cockatrice lair to steal eggs makes for an interesting adventure. For those not heavily armored the risk of this encounter can be quite deadly and trying to actually avoid the confrontation can be exciting.
Cockatrices are generally used as low level opponents. Without the proper spells or equipment an encounter with them can ruin a players day. So any DM should use them with a bit of caution.