From what im seeing , you seem to have a solid grasp on the animals you mentioned . As for your problem , it seems that you are simply too comfortable making sketches and dont make complete work as often . The solution to that is to simply make more completed pieces , even if it wont look as good as you think at the beginning. At one point , you will get comfortable at going forward with detailing your sketches and the clean stuff will feel more ''confident'' . Since there will be less hesitation , it wont be as stiff . Dont forget to critique ur completed piece and remember the mistakes as to not redo them.
Though if the problem are the details themselves, you should simply have a longer session on your animal.
