The cockatrice reassembles a large scaly red or green rooster except for three things: it's large, spiny crest adorning its head, it's tail, splitting in three at the end, and it's beak full of small, sharp, teeth.
It attacks with microscopic poison droplets in its breath which will kill anything (except snakes or toads) instantly.
The cockatrice is a pseudo-dragon rather than a true dragon as indicated by the lack of Draco within its scientific name. However, its scientific name is not of much help with pinning down its ancestry (meaning french halitosis which does indicate that its origins are probably french). A good guess is that it is related to the phoenix which also sports teeth and which in turn is an off shoot of early birds.
It also has a curios relationship with frogs and snakes because although the legend that says that it is hatched from a hen's egg covered by a snake or a toad is pure nonsense snakes and toads do help hatch Cockatrice eggs and are the among the small amount of creatures that it will not eat. This is both because they help cockatrice eggs hatch and because they are peculiarly immune to the venom.