In the Art of Rise of The Guardians the creators admit that one of the characters that changed the most is actually Bunnymund. At first he had to be an upper-crust British scientist , who used a lot of gadgets (like Inspector Gadget) and one of the main problems was his interactions with North, they wanted to make them work like Kirk and Spock from Star Trek (this is actually said by Hamis Grieve, p. 48 in Art of Rise of the Guardians).
But when Hugh Jackman was casted as the one to give him voice, everything fell in place - Bunnymund became an Australian ranger with the use of this fantastical boomerangs. They took of his clothes no robes were cool enough for this rabbit, so they tried to dress him first in Australian Outback slicker and pants... but that didn't fit:
We gave him an Australian Outback slicker and pants, but they really didn't fit him well. Then we figured it out. We were forcing the character to be humanlike, which he didn't need to be. The Tooth Fairy didn't wear a costume, so why did we have to force Bunny to wear clothes? What we really had to focus on was the way he moved, and that led us to the right path.
- Hordos, p. 48 in Art of Rise of the Guardians.
The team begun studying not just rabbits, but also kangaroos, which lead to finding a hybrid between the two spices, to create Bunnymund. While in the new design his posture was more hunched-up, he was able to alternate between using all fours like a rabbit or stand upright like a kangaroo.
The next problem, the animation team was facing was the fact that now that they stripped him from the clothes, did he looked naked? They tackled the problem like the pros they are - Guillielmo del Toro got the fantastic idea and suggested they should give a pelt of fur that actually looked deliberately assembled and not natural, and because del Toro is a Japanese fan, that idea he got it from the Japanese Fur Armor!, which lead Hanenberger to the idea and he gave Bunny a wolf-like hair.
We gave him a big neck and this long, silvery-violet fur, which extends all over his body, just like a wardrobe of his own, we also added some tribal-motif patterns on his body to emphasize this general timeless aura about him, so that you can believe that this bunny is a Guardian and has been around forever.
- Hanenberger, p. 50 in Art of Rise of the Guardians.