I was told that out of the 12 zodiac creatures, making the dog was the most difficult and making the dragon was the easiest.

The dog is difficult because it's the most familiar animal to humans, so people know exactly what a dog supposed to look like. People can get very critical if the dog doesn't look like what it supposed to look like. Details like the curvature of the legs, etc. So they refer to a thick photo book of all kinds of dogs to make sure everything looks real.
Making the dragon is easy because no one has seen a real dragon. So people don't know exactly what a dragon supposed to look like. It doesn't have be so exact like a dog.