The snake-like creature on the box, and on Bruno's Ring was called a Naga. The word "Naga" has its roots in Sanskrit and means "serpent." This was the symbolic sign of the Leviathans, not necessarily a representation of their "true form." The leviathan creature itself, like Lovecraft's Cthulu, were supposed to be indescribably horrible in appearance, enough to "blow your mind" with fear. I do remember that they were slimy, and sometimes left a trail. I agree, fletcher, that the creature may have had something reptilian about it, but I always envisioned it to be like a big, almost shapeless pile of tentacles, perhaps with black eyes. As a kid, my friends and I would imitate the "breathing" of the creature, just for kicks.
Petofi