First i'll show you how to make a fur brush.

Step 1.

You draw a bunch of straight lines with the checkered sort of box I marked in the drawing (Top left), then you fade the edges of the lines to make them look more natural, you do this with the smudge tool. (I also marked that, Bottom left.)

https://i.postimg.cc/Pq9LwdWX/1.png

After you have something that looks like this

https://i.postimg.cc/SRKwbbP9/2.png

You save the canvas in its original save (Not png, JPEG or anything else. If firealpaca/Medibang you save it as .MDP)

Step 2.

Upload the drawing to your art program. You do that by clicking 'Add brush (Bitmap)'

https://i.postimg.cc/rwyghNvd/3.png

Step 3.

https://i.postimg.cc/ZYFr1yP6/4.png
Now this pops up. DO NOT click it away! Instead, change the settings to bitmap watercolor

https://i.postimg.cc/2j21QdF9/5.png

You can then adjust anything you'd like,

These are my settings: https://i.postimg.cc/135hp6BB/6.png

https://i.postimg.cc/ZYcw6QQk/fluff.png
Now you have your own fluffy brush! <3

I hope this helped!

xD