The plural of specimen is specimens.
A specimen refers to a sample or individual unit taken from a larger population or group for examination, study, or analysis. It is commonly used in scientific research, medical diagnostics, and other fields. Specimens can take various forms depending on the context, such as biological specimens (e.g., tissue samples, blood samples), geological specimens (e.g., rock samples), or museum specimens (e.g., preserved plants or animals). They are carefully collected, preserved, and studied to gather information, make observations, or draw conclusions about the larger population or group they represent.
Sources:
https://www.collinsdictionary.com/dictionary/english/specimen