Mudskipper hides are given to Nung, at the Mogre Camp, when Freeing Pirate Pete in Recipe for Disaster, in order to gain access to the crab pen.
The hide is a drop from mudskippers. Contrary to popular belief Mudskipper hides are not used to create Mudskipper hats or Flippers, you must, in fact, kill a Mogre to be able to obtain these