This chapter of Assassin's Creed: Odyssey walkthrough includes the description of all Ainigmata Ostraka located in Naxos. Below, you will find the information that will help you find the Stubborn Mule Ostraka.
Location of Ainigmata Ostraka: Go to Cave of Mt. Zas. Before you collect the item, you need to kill all enemies there.
Ostraka riddle solution: Reach the Island shown in the screenshot above. You will find the item next to the mule.