Source: Star Trek (original series)
Location: Janus VI
Threat Assessment: 4. A silicon creature that looked like either are a large fake rock, a big ugly rug, or the world's biggest pile of fake vomit. It has the ability to dig through rock at an amazing speed. Same ability can burn human flesh and bone. It is fairly tough and is intelligent.
Limitations: At the moment there is only one horta. They have a rather ill conceived reproductive strategy of laying eggs all over the place then dying out save for one guardian. Though tough, enough firepower will wound or kill one.
Personal note: Again, one of those creatures that look much better through the mind of a child. Still got to give the writers some credit I think this is the first silicon based lifeform shown on tv.