* Sharks are fish: "Shark fish" is redundant. Sharks are already classified as fish.
* Koalas are marsupials: Koalas are mammals, specifically marsupials, and are unrelated to fish.
It's possible you're trying to describe something else, but without more context, it's hard to understand your intent.
Could you please clarify what you're looking for? For example, are you asking about:
* A combination of characteristics: Do you want an animal that is as scary as a shark, as cute as a koala, and lives in the water like a fish?
* An animal with a similar role in an ecosystem: Are you interested in finding an animal that plays a similar role in its ecosystem as a shark, a fish, or a koala?
Please provide more information so I can help you!