"The Devil In The Dark"
The Enterprise arrives at Janus VI, where an unknown monster is destroying machinery and killing the miners, threatening the entire mining operation.
"When that creature appears, men die." - Vanderberg (On The Horta)
"There's nothing more dangerous than a wounded animal." - Kirk (After The Horta Flees)
The Horta was a silicon-based lifeform, composed of a material similar to fibrous asbestos. Horta physiology was very different from the carbon-based norm more commonly found in the galaxy. Horta were difficult to detect with tricorders, and were invulnerable to type 1 phasers, though they could be injured with an adjusted type 2. They fed on rock, and thus they were nourished just by tunneling. Horta tunneled through rock like most humanoids walked through air, moving with the aid of an extremely corrosive acid. They left perfectly round tunnels in their wake. This acid was so corrosive that it only left fragments of bone and teeth if used on a human. Although Hortas did not evolve in an oxygen environment, they seemed able to exist in it for extended periods of time.
"The Horta has a very logical mind. And after close association with humans, I find that curiously refreshing." - Spock