Sven- The Monster that Challenged the World (1957)

From Wiki- In the Salton Sea in California, an underwater earthquake causes a crevice to open, releasing prehistoric giant molluscs. A rescue training parachute jump is conducted, but the patrol boat sent to pick up the jumper finds only a floating parachute. One sailor dives in but also disappears. The other sailor screams in terror as something rises from the water.  There are giant molluscs living in the sea, and they drain people dry.  Explosives are used to destroy the eggs, and a final mollusc is shot to death.

This monster was all over the pages of the zines back in the day.  Books would use it as well, since it was a full sized prop.  It is pretty effective, overall, though there are some movements where you can tell it is being pushed or gliding along, rather than walking like a slug should.  The movie has a good cast, and a CO₂ fire extinguisher is always a good way to stop a monster.

Next- Bride of Frankenstein