Monster of the Day: IT (no not that IT)

Source:  It:  the Terror From Beyond Space
Location:  Mars
Threat Assessment:  4.  Incredibly tough to kill.  Has huge claws that can go through metal.  A space suit is no protection.
Limitations:  Tough is not invulnerable.  Hunger for human blood can be used to trick it.  Not too bright.  Could be the last of a dying species.
Personal Note:  Many folks point out that this is the template for Ridley Scott's "Alien."  The suit is scary for a 50's monster but it has always been spoiled for me when it was pointed out that the actor's head didn't fit the mask so they pushed the chin out the mouth and painted it to look like a tongue.