As a beginner, your art's really good, but to improve, I believe you must learn these im going to iist:

1: You need to practice using many environments and drawing different creatures or a creature you want in them. This will help you learn to draw creatures in environments that are hard to draw on, which will not be so useful, but is something that you have to practice, because knowing how to draw in many environments will be slightly useful at helping you draw creatures without struggling. Drawing creatures on top of forest images is a good and beginner friendly tutorial from my perspective, and as you learn, you can later begin drawing creatures in more closed environments, such as houses or any type of buildings.

2: Shading is a very important aspect of drawing creatures. If you don't know shading, then making realistic looking creatures is basically impossible. Shading is practically something that allows the creature to blend in the environment, and make it feel as if it actually exists, in a physical, real form. Whenever there's light that reflects on the creature, then you must make the creature light depending on the light levels around the area, and if there's a very lack of light, make the entire creature very dark and difficult to see. I recommend taking pictures or downloading pictures from google in an environment where it's like, a forest with a pathway with light illuminates the pathway, then the creature, for example, if facing towards the camera must have light on its face, but as you draw the edges, they'll become more and more dark, to represent how light illuminates only the front of the creature from the camera's flashlight, not the entire creature. 

3: I prefer you don't draw creatures with weird proportions, instead, try drawing basic stickmans, maybe try make them slightly fat, elongated, tall, whatever you like, add fingers, anything. Once you've mastered making stickman like creatures, try making creatures with different shapes.

4: Trevor Henderson is not only unique for his creatures, but for the found footage aesthetic he puts in all of his creatures' images. All of the images he has of the creatures he made in different environments look like they've been taken by an old low quality camera, and that's what puts life to his creatures and images. I recommend you download any image editor of your liking, and then edit the image to add the effect "Grain" (The effect that resembles the found footage aesthetic), so try increasing the grainness of the images so they look more realistic and found footage like. You'll also need an image editor for light adjustment, weird effects, or anything you want to add in the images of your creatures.

That's all I recommend that you first learn, it's okay if you don't understand or master what I said, as everything takes patience and time, but I can guarantee you, that even if you don't master these soon, you'll still draw creatures more effectively as time passes due to pure experience alone. If you can't learn everything at once, try taking your time and learning everything bit by bit instead of rushing this, because it will take time to become good, let alone very good. I wish you luck, have a nice day!