Tarrasque

Page 339 of the AD&D 2nd edition Monster Manual Fortunately The Terrasque is a unique creature. The most terrible creature known to inhabit the prime material plane. It is a 50' long biped with a long tail, reflective carapace and two large horns on its head.

The Tarrasque is an eating machine while awake. It will eat EVERYTHING for miles including plants and animals. It's hide is so tough and highly reflective that bolts and rays such as lightning bolt, cones of cold and magic missile do no damage and may (1 in 6) bounce back at the caster. The Tarrasque is also immune to all heat/fire and regenerates 1 hp per round. It is also immune to ALL psionics. Fortunately it is only active for a week or two before it will search for a comfortable lair underground and sleep for 5 to 20 months. Once every decade or so The Tarrasque will stay active for several months, after which it will sleep 4 to 16 years, unless disturbed.

To truely slay The Tarrasque the monster must be killed, and then a wish spell must be employed to seal the creature from this dimension. If this is not done, even the smallest little piece can regenerate to full size!