Drawing out your creatures helps when writing about them because you can look at the image and determine if you're creature could actually do what you're describing it to do or not.
For example, if you create a squirrel like creature. Say it is like a flying squirrel, in reality flying squirrels have a membrane of skin that connects the front and back legs. They use this skin to "fly" across great distances from tree to tree. Truthfully they only glide, but they do have the illusion that they can fly. Now say your creature actually has wings, maybe bat-like wings, so it can actually fly. Perhaps it is also venomous like a snake. Not a nice creature like your average squirrel right? XD
Now we have a basic description of the creature, so we can do a basic sketch.
Isn't it cute? XD Well with those fangs maybe not. Now that we have the sketch we can determine some of what it might do in a story. Like flicking of that big bushy tail when agitated or angry, maybe rubbing it's face with it's front paws or scratching behind its ear with a back paw like a dog. I know, that's basic behavior for such a small creature. Especially, if you've watched squirrels in real life. Things it would do that a real squirrel wouldn't would be leap up into the air and fly over the forest, or strike like a snake to poison either an attacker, or evening meal.
Because of its large eyes you could say it was nocturnal, like a bat would be. It might choose to roost in rocky outcroppings like a bat too.
With the short legs you know it doesn't have a long reach, so it's not going to stretch across a gap to grab something. More likely it would just leap or fly across to get at what it wants. It certainly wouldn't have any trouble getting into your bird-feeder, squirrel stop or not. XD
An image also give you an idea of the size of the creature, whether you add a scale or not. This is a small creature, it's drawn small, and it's based off of something that is small in real life. So it would have the same problems a small creature would face when dealing with larger creatures or large objects. Depending on the situation, and the little bugger's temperament, it might fight or attack even if there is an obvious escape route available.
As you write the story you can continue to reference the sketch. It will help with determining what the creature would/could do in the story as it develops.