Horta anatomy was composed of a material similar to fibrous asbestos. Horta physiology was very different from the carbon-based lifeforms 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 phaser. 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 species possessed an unusually long life span. Every fifty thousand years, all of the Horta died out except for one, the so-called mother Horta, who then watched the eggs until they hatched, and mothered and protected them. Horta eggs were spherical in shape, and they seemed to mostly consist of silicon, aside from a few trace elements. These silicon nodules were stored in a hatchery/nursery inside the Vault of Tomorrow which was accessed through the Chamber of the Ages.
It was in the midst of one of these temporary phases of extinction that the Federation colonized Janus VI in the 2210s. The mother Horta tolerated the Federation presence up until the miners established a new, lower level in 2267, where they first encountered Horta eggs. Thinking them nothing more than balls of useless silicon, the miners' automated equipment destroyed thousands of them. The mother Horta defended her children by carrying out actions of sabotage and murder against the Janus VI colony.
It was only when Commander Spock of the USS Enterprise mind melded with the mother Horta that he was able to determine that the Horta was actually an intelligent lifeform. In fact, before the discovery of the Horta, silicon-based life had been thought a fantasy by Federation scientists.
The mother Horta reached an accord with the miners, who were distressed at the destruction they had caused. The miners would leave the Horta young alone on the lower levels once they began hatching, while the Horta would use their abilities to locate and construct access passages to choice mineral deposits for the miners. Vanderburg, himself, later decided that "The Horta aren't so bad... once you get used to their appearance." likewise, the Mother Horta felt, according to Spock, "that our appearance is revolting, but she thought she could get used to it." Spock further added that "The Horta is a remarkably sensitive and intelligent creature with impeccable taste," after sensing that she found his ears to be "the most attractive Human characteristic of all."
In 2268, when considering the possibility that the rocks on the planet they were investigating were alive, Sulu reminded Kirk of "Janus VI, [and] the silicon creatures." McCoy then further reminded Sulu and Kirk that their instruments had registered the Horta as lifeforms, unlike the rocks they were surrounded by now. (TOS: "That Which Survives")
Information about the Horta was displayed by a computer, as an okudagram graphic depicting Comparative Xenobiology, in Keiko O'Brien's schoolroom on Deep Space 9. (DS9: "A Man Alone", "The Nagus" okudagram)
The German word "Hort" (male gender, der Hort) means "hoard" (as in hoard of treasure or supplies) derived from Latin "hortus", garden. Recently, the word's meaning is extended to include "all-day nursery" – a place where you "hoard" children (hence cognate to Kindergarten), fitting after-the-fact the role the Horta had.
"We made a "spec" [note: industry idiom for an unsolicited pitch without any guarantees] deal with Janos. If he came up with a really great creature for a script Gene Coon was writing, we'd rent it and hire him to play the part. Janos was back within a week's time with his custom-designed creature. It was a large pancake-shaped glob of gook with a thickened raised center and fringe around its circumference. It sure didn't look like much. As Janos took the glob out of sight to put it on, Gene Coon raised an objection "Bob, why are wasting time with this?" Suddenly, the blob skittered around the corner, making straight for us. Then it stopped, curiously, backed away, and rotated in place. The blob gathered itself up, quivered, made a whimsical up-and-down movement, grunted, and skittered away again – leaving behind a large, round white "egg". Coon was dumbfounded. He watched the creature giving birth. And when the creature suddenly turned and scurried back to nuzzle its "child", Gene was sold. "Great!" he exclaimed, "It's perfect! Just what we need." Then he excitedly hastened back to his office to finish writing the script. "Gene Coon's "The Devil in the Dark" became one of Star Trek's most famous episodes. And Janos Prohaska played his own creation, one of Star Trek's most famous creatures, the highly imaginative and custom-designed mother Horta." (Inside Star Trek: The Real Story, pp. 214-215)
What neither Justman nor Coon had realized at the time however, was that Prohaska had actually already created the creature previously for the original ABC series The Outer Limits, first appearing in the final episode, and that he had only slightly modified the rubber costume with veins and the "fringe" for its Star Trek appearance. (Star Trek: The Magazine Volume 3, Issue 9, p. 73) Titled "The Probe" (with Peter Mark Richman), the Outer Limits episode's storyline was about survivors of a plane crash in the Pacific waking up to find themselves (and their life raft) on the floor of an alien spacecraft sent to collect terrestrial lifeforms. In this episode, broadcast in January 1965, the future Mrs. Horta was performing yeoman service as a giant cold germ threatening the hapless Earth people. (The World of Star Trek, 3rd ed., p. 74)
In an interview with John Ellis for the DVD commentary of the Steve Canyon episode "The Search" for the DVD collection Bob Hoy revealed to Ellis that he actually was in the Horta costume in the same episode (in certain shots as a "stunt performer") and that Janos Prohaska, who created and also wore the costume, was a long time friend and coworker. 
According to the audio commentary for "How Sharper Than a Serpent's Tooth", David Wise mentions that for Kukulkan's zoo, "If you were to read the original script, we had animals, various life forms from earlier episodes of the live action Star Trek. There was a tribble, a couple of tribbles in one cage, there was a Horta from "Devil in the Dark" [sic] in another cage, they were supposed to be reference, the various menagerie of characters who had appeared in earlier Star Treks."
A Horta was referenced in the first-draft script for "In Thy Image" – the story that gradually developed into Star Trek: The Motion Picture – in which Kirk reminded Dr. McCoy, now a veterinarian, of the Horta having been "patched up" by McCoy using silicone cement. (Star Trek Phase II: The Lost Series, p. 125)
Two days before filming of the Federation Council scenes in Star Trek IV: The Voyage Home, it was requested that one of the aliens present in the setting be a Horta ambassador. As notice of the request was given so soon before the scenes were shot, however, the creation of a Horta in time for filming was an impossibility. Thus, the Horta failed to make an appearance in the film. (text commentary, Star Trek IV: The Voyage Home (Special Edition) DVD)
Some new CGI visual effects were created for the Horta, in the remastered edition of "The Devil in the Dark", for the 40th anniversary of the original series. Notably, the Horta emerging/tunneling through a particular rock face, when Captain Kirk encountered the Horta mother on his own, was a combination of new and original footage.
According to several novels and comic books by Diane Duane, there is a Horta crew member on the Enterprise, Ensign Naraht (β), one of the several thousand children of the Horta who appears in "The Devil in the Dark". In particular, Naraht plays a critical role in The Romulan Way. According to that novel, Horta who enlist in Starfleet must be regularly spray-coated with teflon, since oxygen-based atmospheres are highly caustic to silicon-based lifeforms. Devil's Bargain has them cooperating with the Federation mining colony Vesbius to de-stabilize an asteroid headed for the planet so the Enterprise can safely destroy what is left of it. Some of the Horta ask for Kirk's recommendation to Starfleet Academy. The book also suggests that Horta can live in the vacuum of space for a short period of time.
The novel Articles of the Federation, by Keith R.A. DeCandido, has the Horta as members of the Federation as of 2380 and are represented by one Councilor Sanaht (β). Further appearances include the Greg Cox and John Gregory Betancourt's DS9 novel, Devil in the Sky. Hortas were also mentioned in The Lost Years.
In the TNG novel Dyson Sphere, it is revealed that Starfleet has starships crewed entirely by Horta. These ships are of standard design, with nearly all amenities removed, and are filled with solid stone, which the Horta can reshape as they see fit.
In the MMORPG Star Trek Online, in the mission "Mine Enemy", a Horta kills Tal Shiar officers and burns the words "NO KILL I, NO KILL I" into a cave floor with acid. If the player observes this, he or she will get the accolade: NO KILL I. The mission reward is a Horta hatchling pet that follows the player when activated from the inventory. Horta are also seen being employed for mining operations, as a player's fleet dilithium mine has Horta miners working in environments hostile to humanoid lifeforms. There you can also purchase Horta duty officers for your ship. Also, during the numerous anniversary events, a Horta is one of many creatures that players could be temporarily turned into by the visiting Q should they annoy him, or just by asking if he really is that powerful.
The short story "Guardians" from the anthology book Strange New Worlds VII featured a number of Horta being relocated to the Guardian of Forever's planet to protect it from races or beings who might seek to abuse its powers.
Devil's Bargain provides some insight into Horta culture. The mother Horta is referred to as the All Mother. In addition, Horta belong to clans based on their abilities and roles. Horta names are descriptive of the individual's strengths, abilities, or sometimes bestowed on them by others.