The Basilisk is a mythological creature which has shown up in many bestiaries and fantasy works. Described as being hatched from the egg of a snake (or a toad) by a cockerel, it is generally portrayed with a mix of reptilian/amphibian and bird features. This makes it similar in appearance to its cousin the Cockatrice, a dragon with birdlike features which is said to be hatched from a chicken egg (sometimes a rooster egg) by a snake.
Whether these are two distinct creatures or whether they’re interchangeable names for the same thing is something of a Cyclic Trope.
Originally (i.e., in the Roman Empire and the early Middle Ages), the basilisk was depicted as a horribly venomous snake, while the cockatrice, which first shows up in medieval England, was a chicken-reptile chimera that turned people to stone with its gaze. Eventually, the two creatures blended into a single myth for a number of reasons, and became synonymous terms for a single monster. In more recent times, some (although not all) works of fiction (possibly inspired by Dungeons & Dragons) have come to once again set them apart. The basilisk tends to be shown as a more distinctly reptilian snakelike or lizardlike animal, and usually maintains a deadly petrifying gaze. By contrast, the cockatrice tends to be portrayed as more distinctly avian, with a largely birdlike body bearing a snake tail, bird legs, and wings capable of flight.