Predators from Manark IV that look like lifeless rock crystals until the moment they attack, mentioned during the landing party's captivity among the Vians in The Empath.
The sandbats of Manark IV are ambush predators with a remarkable disguise: at rest they appear to be inanimate rock crystals, and they give no sign of life until they strike.
They are mentioned in The Empath, while Kirk, Spock and McCoy are held in the underground laboratory of the Vians on Minara II. Faced with captors and specimens that stand unnaturally still, the landing party is reminded that apparent lifelessness proves nothing, and the sandbats are offered as the example.
The creatures are not seen on screen and nothing further is said of Manark IV. The reference is typical of the original series' habit of sketching a wider galaxy through passing remarks.