The great thing about SF is that we can invent all sorts of cool monsters. One way to do that is to take an ordinary creature or object from Earth and make it a lot bigger and more malevolent. For example, consider this little cutie:
Aww... Adorable, right? Now let's make him bigger and meaner:
Yikes! Same critter, basically. Both are excellent predators. But one is cute, and the other will eat you.
The problem with big cats is that, while dangerous, they're also kind of ordinary. They usually don't make for good villains, though there are some rare cases.
So it's better to go with something extraordinary... something with which most people aren't familiar. Like single-celled organisms. Imagine this creepy white slime-mold stuff:
...but on a much larger scale. That's basically the plot of The Blob, by the way. The blob was just a large amoeba that had an appetite for man-flesh. And it was pretty horrifying, too. When I saw that movie, it scared the crap out of me. I was just a kid at the time, so maybe I'd have a different reaction seeing it as an adult, but still...
Here's a real-life amoeba feeding on a couple of doomed single-celled organisms. Watch the poor victims thrash with pain and terror, and imagine they're humans:
The cool thing about protists is that there are all kinds of different types. You can mix and match their characteristics and come up with something completely new. You could start here, for example, and let your imagination run wild with the possibilities.
Now, taking small things and embiggening them is hardly a new idea. In fact, the B movies of the 1950's were full of such monsters. And they were often cheesy as hell. But that's a flaw of execution, not principle. The special effects simply weren't there to do it right, and the scripts and acting were often sub-par. Compare the giant spider of Tarantula with the one from Return of the King:
Big difference, right? A giant spider doesn't have to be cheesy or campy. It's all in the execution.
So there you go. Quick and dirty monsters. Take the strange, and make it big and bad. And then twirl the tip of your mustache and laugh evilly, because that's what good monster-makers do. :D