I started writing a short story about flying dogs awhile back. The story is a work-in-progress, but I hope to finish it soon and post it on Wattpad.
In preparation for its book cover, I had to first practice sketching and painting flying dogs. Here was the result of my practice. This was not solely from imagination; I relied on a number of reference images.
There are certain objects that I'm good at drawing from imagination, because I've drawn them over and over. Ice cream, trees, architecture, rainbows, and flowers come naturally to me without much effort. I consider these objects and more to be part of my mental library of easily-drawable shapes.
There are also things that I lack the skill to draw confidently without a reference image, because I haven't drawn them enough times to commit them to memory. It really is just a matter of practice, though. I'm hoping that with time and daily study, I'll be able to expand my "library" significantly.
Dogs are one of those things that I require reference images for, but hopefully that will change soon. If I rely solely on my imagination, there are a couple of default dogs that I can depict repeatably on paper, but none of them are flying dogs. This practice session hopefully fixed that problem, though. We'll find out when I try to draw the final version.