But it has scales? And I doubt Solas ran back to Skyhold before the inquisitor just to paint some ambiguous clue at the end of a mural dedicated to her deeds.

These are the reasons I think this is a wolf.
Ears - dragon horns will be bigger when compared to his head.
Curl - the curl under the right figures chin can easily be a fur curl, just like the one on the left.
"Scales" - both drawings have open triangles, in the right one it looks a bit like scales, but it is exactly like what appears on the left.
Paw - both creatures have paws, a dragon would've had claws.
Besides all that we need to remember that this is a sketch, things like texture are added with coloring and brush strokes, the sharp edges would become softer when painted. I do think the creature on the right looks dragonish but I believe it's because
1) It is the Dread Wolf, it wouldn't look like a normal wolf, it would look fiercer.
2) The Dread Wolf is an elven god, gods and dragons in this world seem to be connected. Flemeth can become a dragon because of Mythal's spirit, Mythal is depicted as a dragon or a lady with dragon wings, as well as other gods.
3) It could be that the dragonish part was added to express the fact that the Dread Wolf now holds Mythal's powers.
Edit: I tend to go with 1 & 2 since I want to believe Solas didn't plan on stealing Mythal's powers before the battle. These are just theories that seemed plausible from this thread.