So sketching ideas for creatures is an evolutionary process and one that you should work through yourself and try and get to grips with quick sketching your ideas down on to paper as fast as you can, so you can capture the energy of your own ideas.
The sketching process is a very methodical process that you begin right away. And it's up to you to make use of good ideas and get rid of bad concept ideas. The process of sketching helps with this and the elimination of ideas you don't like so that you can develop your better ideas further.
In the following drawing video I draw an idea of an arm that is a monster concept I was challenged to draw by another YouTuber. And this video is a prime example of drawing a creature concept from a basic arm shape.