"Is there a cat"
* This asks if a cat exists in a specific location or situation. For example:
* "Is there a cat in the house?"
* "Is there a cat that can catch mice?"
"Any cat"
* This implies a general question about cats, and it usually means any cat will do. For example:
* "Can I pet any cat?"
* "Do you have any cat food?"
To summarize:
* "Is there a cat" is asking about the existence of a specific cat.
* "Any cat" is asking about the availability of any cat, regardless of specific characteristics.
Let me know if you have a specific sentence in mind, and I can help you figure out which phrase is more appropriate.