I took a couple of days to consider my choice because I was stuck on the 'What Monster scared you as a kid?' criteria. For me, 'kid' is anybody before they hit 13. There were a few monsters that scared me before I reached adulthood, for instance
James Carpenter's The Thing, but the criteria knocks those out of the running in my opinion.
Then there is the definition of monster itself. My all-time horror movie remains
The Exorcist. Some people label Regan as a classic monster. I have said so in the past as well but I do not always see Regan as the monster in that film. I see her as The Devil personified. A.K.A. evil incarnate. Many movie monsters commit evil acts but Satan does what comes natural, lol.
All of the above narrows my choice to one monster. And it is a monster that does not usually scare monster lovers:
The Blob
I was around seven when I first saw Steve McQueen's 1950-something original during a sleepover at my grandmother's house. It came on television as a late night offering around 10. My eyes were glued until it ended and the tv station said goodnight (younger folks cannot relate, lol).
When I was a few years older, my mother took me with her to the theater and watched the 1972 remake. I thought I had gotten braver since my first time seeing that... thing. ha.
The Blob could squeeze through
ANY nook, hole or cranny. That is an old adage but it was literally true. It was a gelatinous mass, whose
sole motivation was consuming living creatures. (Note: I was still young and did not stop and think why this thing did not consume trees, bushes, etc., lol).
Zombies did not have anything on The Blob. Zombies ate brains. They might eat your flesh if it was part of their diet. Glutenous zombies might even eat your entire body but then it takes itself out of the terror equation because it would be too full to move again.
The Blob? Oh. It will squeeze itself through plumbing to get you. It will slime underneath a door or window sill. "Well, it is just goo right, DE?" Yeah. Sure. It was goo that if even a drop of it stuck to your clothes or skin, you are dead. Forget dead. It would spread over your entire body. Afterwards, it would immediately dissolve everything. Flesh. Bones. It did not care. The more it ate, the larger it grew. Plus, it had the capability of moving
FAST. One second, it is resting fifty yards away. The next second. Well. You are lunch by then.
Cold was its only weakness.
Freezing turned it inert.
Just typing all this brings back fond memories. Excuse me while I go stuff a beach towel underneath my office door now.