The best substitute for "specimen" depends on the context. Here are some options, categorized by their specific meanings:

General:

* Sample

* Example

* Instance

* Exhibit

* Model

Biological:

* Organism

* Creature

* Subject

* Test subject

* Tissue

Medical:

* Biopsy

* Culture

* Smear

* Slide

Other:

* Item

* Piece

* Object

* Thing

To choose the best word, consider:

* What kind of specimen is it? (biological, geological, etc.)

* What is the purpose of the specimen? (study, display, analysis, etc.)

* What is the tone of your writing? (formal, informal, scientific, etc.)