Um, thinking vegetable? Doesn't sound all that scary, but in The Thing from Another World (1951), that is exactly what a small group of scientists and airmen are up against at an isolated North Pole base.
James Arness, who found later fame as Marshall Matt Dillon in Gunsmoke, is the monster, a thinking vegetable man from another planet. The eponymous Thing feeds on human blood, creepy, sure. Realistic, heck no. But it does echo back to H.G. Wells's War of the Worlds.(Remember what the Martians used human blood for in that book?)
The Thing might be a 'B' movie, but it is about as tense and fast paced as it comes for 1950s sci fi/horror. Fans of vintage Frankenstein films will recognize a bit of Karloff's make-up influence in Arness's costume. The Thing from Another World was remade in 1982 by John Carpenter (simple titled The Thing), a frightening film in its own right. Both movies were based on a short story by John W. Campbell, Jr., although the 1982 version is more faithful to the source material.
The movie ends with the famous last line, "Keep Watching the Skies", a nod to cold war tensions and the fear of the unknown.