Seeing the monster in a horror film nearly always ruins it.
Most monsters are crap and not that scary to see. Seeing them reduces any mystery that might have been granting them extra power over the viewer. The viewer tends to naturally apply their own worst fears onto the monster when they lack more information about it. Once they’re given the concrete information, the thing is naturally less frightening.
Fiction writing has an easy advantage over films in this respect, as more of the monster is left to the reader’s imagination.
The point is similar to “hopes and fears”, in that potential is more interesting than concrete conclusions.
One notable exception to this might be the xenomorph in Alien. It manages to remain scary even when we do see it, so clearly that’s not an impossibility. That achievement might be due to still not giving the viewer a good, clean look at the xenomorph for more than a brief moment, or due to the xenomorph being a person in a suit and not crappy-looking graphics. It might also be due to the artistic design of the xenomorph that’s designed to play on a set of basic fears, which channels well through the screen.