Guinea pig

Guinea pigs were small Earth mammals often used by humans for scientific experiments. Because of this, the term "guinea pig" also came to mean any test subject and not specifically an actual member of the species. 

The USS Enterprise (NCC-1701) kept guinea pigs in the ship's laboratory. In 2266, when the Enterprise approached the planet K-G, plant spores penetrated the ship and caused the guinea pigs to transform into tree-like plants. They attacked Spock and Dr. Leonard McCoy, but were destroyed by security guards.