The Horta are silicon-based lifeforms from Janus VI. It is composed of a material similar to fibrous asbestos.
Their physiology is very different then carbon-based lifeforms. Tricorders can barely pick them up and Phaser Type-1 have no effect on them.
They feed on the rocks on Janus VI. When they move they have the aid of a corrosive acid.
They live for 50,000 years and all but one dies out. The last one will watch the eggs and protect them.