WoWDB

All of the rare mobs listed are accessible in any Stage of the  Isle of Thunder except the following:

• God-Hulk Ramuk - Requires Stage 3 or higher
• Backbreaker Uru- Requires Stage 4 or higher
• Lu-Ban - Requires Stage 4 or higher
• Molthor - Requires Stage 5 or all Stages completed

Additionally, there is one other rare mob on the Isle of Thunder who is not listed, Ra'sha. He does not require a specific Stage.

Note that even if you can get to the location where the rare mobs are supposed to spawn, if the Isle of Thunder is not in the right Stage, the rare mob will simply never spawn (and neither will any Trove of the Thunder King in that area).