What Pokemon is in Hippowdon Temple guarding the Yellow Gem?

There is no Pokemon called "Hippowdon Temple" or a "Yellow Gem" in the official Pokemon games. It's possible you're referencing a fan-made creation or a specific region in a fan-made game.

To help you, could you please provide more context? For example:

* Which Pokemon game are you referring to?

* What is the context of this "Hippowdon Temple" and "Yellow Gem"?

* Is this from a specific fan-made game or a specific fan-made story?

With more information, I can help you find the Pokemon you're looking for.