Honestly, deriving inspiration from other works/folklore/real-life is one of your best bets. Think about the role that each creature plays in your story, and what it would need to fulfill those roles.
Is your creature meant to be scary/creepy? Look at some horror or thriller monsters. What makes them scary? Take those aspects of the creature and apply your own spin to it.
Is it meant to be a massive, hulking animal? Watch how real large animals move around and interact with their environment. There's a certain weight and stockiness to everything they do. Perhaps find a series that deals with giant monsters and see how they did it.
The same mentality can be used to create just about anything with a bit of imagination. Plenty of real-life creatures are fantastic references to draw from.
Make sure the creature/creatures you create make sense in their world. Are they supposed to be elusive monsters? Or are they common animals? Reflect that in their design and biology. Are there predator/prey relations? If so, how does the predator hunt, and how does the prey protect itself? Think about these things, even if you don't necessarily explain or expand on them in your actual story.
That's all the advice I can really give for now.