Initial thoughts
Hellhound is a well known mythology creature. One example is Cerberus in Greek myth. Descriptions of Cerberus vary, including the number of his heads. Cerberus was usually three-headed, though not always. But Cerberus is not the only one, the forklore all around the world often has something similar such as Black shuck.
I guess since human being domesticated dog 30,000 years ago, its figure influnced our story content through the history. No matter the variations of different type of hellhounds, it is always black and it may have multiple heads. However, the idea for this painting is that even the hellhound is scary or ferocious, it was born as a puppy once upon a time. Back then it was small, playful and maybe vulnerable, just like all dogs. In Greek myth, Cerberus is the pet of Hades, the king of the underworld. So, I want to show what the hellhound would look like when it was a pet puppy.
Digital sketch
In the digital sketch I made the hellhound’s heads bigger than the body. I have to change the size and ratio in the final painting. It is hard to put all three heads on the same shoulder. Three heads means three necks and all necks merge into one. Thus the hellhound has a wide and broad chest. I also add the shape of two rare legs in the actual painting, otherwise the puppy looks weired as if the rare half of its body is gone.
Considering the hellhound’s master, I choose the volcanic landscape as the background. I think the puppy doesn’t mind the lava at all. Similar as Targaryen’s dragon blood thing, fire comes cold to dragon. Here it would be the lava comes cold to the hellhound.
And the human hand skeleton, yes, it is the size comparison and gives the puppy some character. That is its fetching toy! I assume it can find lots of that toys in the underworld.

Summary
When talk about three heads monster, I often use Ghidorah as my reference because it has three long necks to separate three heads well. Ghidorah also has significantly wide chest and shoulder distance in order to merge the three necks into one body.
The is one of antomy nightmares……another one would be the two sets of rib cages (two sets of heart and lung) of Centaur. 🤣

