MrCrumb made an announcement of a new skin dedicated only to Amumu lovers:

In this newest patch, everybody's favorite sad mummy will be sporting some stylish Egyptian threads. Add this skin to your inventory to give Amumu the regal look he deserves.
Once the patch has been released, click on the Skins tab in the Store to purchase and unlock the Pharaoh Amumu skin.

Source: Pharaoh Amumu Skin Available in the Upcoming Patch Thread