I've watched a lot of Sci-Fi in my day. Many of these shows have monsters. As a matter of fact, I just finished watching Netflix's 'Stranger Things,' with a monster that will bring nightmares to almost anyone's dreams.
What makes a monster? To me, there are two types of monsters. A monster can be gruesome, horrible, and really scary. Or it could be a kid down the block who looks completely normal. The look of the monster isn't as important as its actions.
The actions have to instill terror in the heart of the viewer. They can never know what horror is coming next, or what dark evil lies in the creature's heart, if it has one.
The best monsters start out as mild, even tempered, but slowly become worse and worse. That can be shown by only alluding to the monster and not show it to begin with, finally letting the viewer see the monster for what they really are.
I remember shows from The Twilight Zone that included monsters. The one that got me the most is the kid with powers -- 'It's a Good Life.' Here's a synopsis from that link:
Six-year-old Anthony Fremont looks like any other little boy, but looks are deceiving. He is a monster, a mutant with godlike mental powers. Early on, he isolated the small hamlet of Peaksville, Ohio. In fact, the handful of inhabitants do not even know if he destroyed the rest of the world or if it still exists. Anthony has also eliminated electricity, automobiles, and television signals. He controls the weather and what supplies can be found in the grocery store. Anthony creates and destroys as he pleases, and controls when the residents can watch the TV and what they can watch on it.
The adults tiptoe nervously around him, constantly telling him how everything he does is "good", since displeasing him can get them wished away "to the cornfield", where they are presumably met by a less-than-happy ending. Finally, at Dan Hollis' birthday party, Dan, slightly drunk, can no longer stand the strain and confronts the boy, calling him a monster and a murderer; while Anthony's anger grows, Dan begs the other adults to kill Anthony from behind -"Somebody end this, now!"- but everyone else is too afraid to act. Before Dan is killed, he is shown, indirectly by his shadow, transformed into a Jack-in-the-box.
His widow breaks down, but no matter what happens, the people of Peaksville make sure to think only good thoughts and repeat "That's a real good thing what Anthony did!" and "It's a good life."
It's horrifying to think that just a young boy can be so evil. That's what makes it even scarier, to me, than having the first type of monster, one that looks evil.
So when you write about a monster, don't reveal all of its evil qualities all at once, but little by little, make it more evil as it goes. It's like a secret evil that the reader doesn't know about until the very end, but suspects as you go along.
Have a great day!