Dragons were different from humans. They could indeed change into their human form and appear as if they were no different from humans, but even if they looked very similar to normal humans from the outside, their true forms were not similar to humans at all.
On the other hand, dragons' real forms were more similar to snakes but much larger than ordinary snakes. Perhaps a normal snake could only grow to two or three meters in length, but an adult dragon would be able to reach a size of twenty or even thirty meters in its lifetime.
They did look like snakes, and that was why they could move on the ground like ordinary snakes. But, while snakes had no legs and could only move on the ground using their bodies, dragons were different in that they had four legs which would help them walk on the ground like four-legged animals.
