Gleeoks are the three head dragons (not the spirit dragons). One is on the giant bridge above lake Hylia.

Gleeoks appear in specific spots only, and there aren't very many in the game. They're more rare than Hinox. When I first said hello to one, not knowing what its attacks were, I died within seconds. I got stun-locked and murdered. So, if you see a giant, three-headed dragon just standing around, save your game before introducing yourself.