Brainstorm the type of monster that you want to create. Research past monsters to grasp the different types of monsters created and what makes them "monsters" in the first place. Websites like Monstrous.com have a large database filled with different types of monsters categorized into different sections like "Zombies" and "Dragons."
Use animals as the basis for a monster. Whether you choose to mutate an animal or select certain attributes that can be applied to your monsters, specific animals can showcase this and give you more ideas. UMich.edu hosts an animal classification website that categorizes and describes hundreds of animals. Browse descriptions and pictures to get more monster ideas.
Create a hybrid animal for your monster. For example, you could mix a fish and gorilla to make an aquatic monkey-like animal. This is one of the simplest ways to create a monster because it is not something normally seen in nature. You can mix more than 2 animals into a hybrid for a more creative monster.
Write out a list of basic body parts from head to toe and leave a blank space next to each part. Fill in the spaces with specific attributes that you want your monster to feature. For example, for the eyes you could put "piercing green eyes with small pupil." This process will help you visualize your monster better.
Create a list of strengths for your monster. For example, does it have perfect vision, can it fly or does it have super strength? Other strengths could include stealth, speed or protection, like a natural coat of armor.
Create a list of weaknesses for your monster. Think of real animal weakness like the slowness of a turtle, small size of an ant or small brains of a fish. The monster could also have psychological weaknesses like anger issues. Weaknesses are crucial to monster development because that is what makes them relatable or destructible.
Draw a sketch of the monster to plan out the final visual aspects. Draw the monster in different states like posed, in action or even sleeping. This will help you visualize its physical attributes and movement style.
Write an origin story for the monster. Where did the monster come from? Was it scientifically created or was it created through nature? There are endless possibilities for this, and even if you do not use this back story within your project, it will help develop your monster's character.