Please read what was posted above:
"Spawn mechanism: Spawns 60-85 minutes after the death of any griffon or pegasus on the island. Will despawn after a while. (how long?)"
You ONLY need to KILL ONE griffon or pegasus. And yes, sometimes he fails to spawn. Sometimes he fails to spawn when you kill them all also!!!!
Killing only ONE griffon or pegasus has 2 major positive sides.
Firstly, you leave the door open for the next guy coming after you. They won't have to wait for a chance at a spawn. (You can do multiple respawns for yourself or boxes as well!)
Secondly, it does mess up. Sometimes the keeper doesn't spawn like he should. If he doesn't spawn 85 minutes later, go ahead and kill ONE more griffon or pegasus.
Exact spawn loc I don't have, but it's off to the side a ways from the building. If I recall correctly (guessing) if facing building where you lead into it, go behind it and to the left corner of the island. If I get an exact loc I'll let you know.
Please follow my advice as it will save you time in the long run and help out the next guy.