Devilsaur Leather is the leather of dinosaurs that appears similarly to a Tyrannosaurus Rex. Devilsaur can only be skinned form the dinosaurs in Un’Goro Crater with the name Devilsaur and King Mosh. The best way to farm Devilsaur Leather is be camping Devilsaur Queen located south west of Un’Goro Crater. It is a level 55 Elite that will respawn ever 30 seconds or so.
See below for the exact location of Devilsaur Queen to farm Devilsaur Leather in WoW.