A tnucipen was a type of small animal.

In 2269, during the battle between Spock and Spock One on Organia, Captain James T. Kirk (due to the effects of the thought-shield) saw the drowned bodies of a wide variety of small animals, including tnucipen. (TOS novel: Spock Must Die!)

Tnucipen were presumably named after the Tnuctip from Larry Niven's Known Space series.