The Leviathan is an immense sea creature with a long serpentine body coiled in rings.
In Isaiah 27:1, the Leviathan is described as the "piercing serpent" and the "crooked serpent."
In Job 41, he is described as a monster with terrible teeth, and scales that fit so tightly together that not even air can pass between them. He breathes fire and sneezes light, his eyes shine red like a sunrise, and he can make the sea boil. There is nothing on earth that doesn't fear him.
In rabbinic literature, God first created both a male and a female Leviathan but then realized that their offspring would destroy the entire world so he killed the female and set her aside to use later as food in a feast celebrating the advent of the Messiah. At that time the male will also be hunted down and killed for food.
According to R. Johanan, the Leviathan is a fish with horns that have an inscription across them reading "I am one of the meanest creatures that inhabit the sea. I am three hundred miles in length, and enter this day into the jaws of the leviathan."
The body of the Leviathan glows, especially his eyes, when he is killed his hide will be used to create so many wall decorations that the entire world will be illuminated.
There is only one creature that the Leviathan fears, a small worm called a "kilbit" which can kill large fish by clinging to its gills.
See Also: Creatures by Type » Dragons, Sea Monsters, Serpents
Jewish Encyclopedia. Leviathan and Behemoth. URL: http://www.jewishencyclopedia.com/view.jsp?artid=275&letter=L. Retrieved on 03/14/2010.
The Holy Bible, King James Version.