Film: Space Master X-7 (1959)
Location: Civilized area
Height/Weight: Constantly fluctuates.
Summary: Do you ever wonder why Mars is red? Is it because there was a great cataclysm that left it that way? Is it just because? Why no, it's because a giant fungal rust monstrosity makes its home there. Who wouldn't've guessed?
History: After the satellite XN-712 crashes back to Earth from a trip to Mars, a scientist discovers a most unusual stowaway: microscopic fungus that seemed to be growing and growing with every bit of protein it devoured. The doctor would dub it "Blood Rust" based on its color. Unfortunately, the rust grows to ludicrous size, and even after the house is burned down, the rust finds a new host in the form of the scientist's estranged lover, who unintentionally infects all that she touches.
Notable Kills: Blob of '58 did it all better, just so you know.
Final Fate: After the rust finds its way onto a plane, the people scramble to land it safely. Upon doing so, everyone gets decontaminated by the authorities, ending the rustic threat.
Powers/Abilities: The rust is constantly growing with a steady supply of food, and can spread at an alarming rate.
Weakness: Fire seems to be a viable option, though simply decontaminating yourself works better.
Scariness Factor: 4-While it's pretty easy to undo the damage done by it, the rust could have easily spread on a pandemic level if no one had caught up to that woman, and getting absorbed by space fungus doesn't sound like fun, does it?
Trivia: -Rusting in space does indeed happen, but only rarely, and mostly when something is close to a planet.
-The entire point of this film is said to be the need to cash in on the launching of the very real satellite known as the Explorer 1.