You could look online or in books at imaginary creatures to get inspiration and then draw your creature. Label your picture highlighting the different body parts, maybe the types of teeth it has and you might want to think about it's skeleton!
You will need to think about where your creature might live and why? It might help to research real creatures to get some ideas!
Think about what your creature might eat and what might eat it, you could even draw a food chain to show this!
You could use what you know about food chains and eating habits (carnivore, omnivore, herbivore) to write a daily menu for your new creature.You could also make the habitat and the creature using paper, card or playdough.