"For by art is created that great Leviathan called a Commonwealth or State... which is but an artificial man, though of greater stature and strength than the natural, for whose protection and defense it was intended; and in which the sovereignty is an artificial soul..."
The Leviathan is traditionally a biblical figure:
"Canst thou draw out leviathan with an hook? or his tongue with a cord which thou lettest down?... Will he make many supplications unto thee? will he speak soft words unto thee?... Canst thou fill his skin with barbed irons? or his head with fish spears? Lay thy hand upon him, remember the battle, do no more... None is so fierce that dare stir him up: who then is able to stand before me?"
Job 41: 1-10 (KJV)
"Canst thou draw out leviathan with an hook? or his tongue with a cord which thou lettest down?... Will he make many supplications unto thee? will he speak soft words unto thee?... Canst thou fill his skin with barbed irons? or his head with fish spears? Lay thy hand upon him, remember the battle, do no more... None is so fierce that dare stir him up: who then is able to stand before me?"
Job 41: 1-10 (KJV)
Here, the Leviathan is a symbol of God's all-consuming power, which we as humans are unable to comprehend. Hobbes chose the Leviathan to represent the unyielding power of a united commonwealth, as well as the sovereign/god who ruled over it.