I can't believe you remember the best monster ever! It was a big rubbery slug-thing that would dig a hole, pull a monster skull over the hole, and coil itself up like a spring to lay in wait. Prey would walk by and BOOM! SPRINGING SKULL OF DEATH!
I've actually DMed a few of these over the years. They rock.
(instead of the rubbery bit, make it a sort of skeletal snake with the skull as its head, and give it a hypnotic dance [all who view save vs. hypnosis] as well as its ordinary attacks...tons of the fun)
It's basically a gelatinous cube that looks like a wall. Usually stone. They paralyze you. And smell like vinegar.
