It! The Terror from Beyond Space (1958) Edward L. Cahn

In 1973, the first manned expedition to Mars is marooned; by the time a rescue mission arrives, there is only one survivor: the leader, Col. Edward Carruthers, who appears to have murdered the others! According to Carruthers, an unknown life form killed his comrades during a sandstorm. But the skeptical rescuers little suspect that "it" has stowed away for the voyage back to Earth...

It! The Terror From Beyond Space is simply one of the best B-movies from the 50s. This film would actually be considered a classic (rather than just a cult classic) if they just took out one of the parts of the movie where the zipper on the creature costume's back is clearly visible for about two seconds. The idea that inspired the later-on Ridley Scott thriller ALIEN (1979). A crew of astronauts from Earth are on their way home from Mars, where one of their crewmen was later strangely found dead after they went exploring. On their way back, they realize that a horrible Martian creature that feeds on blood has stowed away on their ship, and they won't be reaching Earth for DAYS!!! Highly thrilling and enjoyable.