More Formal:
* A figment of the imagination: This emphasizes the unreal nature of the thing.
* A creation of the mind: This highlights the process of creation.
* An imaginative construct: This emphasizes the mental construction of the thing.
* An intellectual fabrication: This emphasizes the deliberate, non-physical nature of the thing.
More Informal:
* A brainchild: This implies something unique and original.
* A fantasy: This emphasizes the unreal and whimsical nature of the thing.
* A dream: This suggests a more personal and emotional connection to the thing.
* Something made up: This is a simple and direct way to say it.
The best alternative will depend on the specific context and the tone you want to convey.