A reepicheep 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 reepicheeps. (TOS novel: Spock Must Die!)

Reepicheeps were presumably named after the character Reepicheep from C.S. Lewis' Chronicles of Narnia series.