The lexical unicorn zoo just keeps on adding exhibits. Unicorns or unicorn companies are startups valued at over a billion bucks before going public. Job seekers yearn for their unicorn job, while freelancers seek a unicorn fee. NBA All-Star Kevin Durant recently praised the skills of New York Knicks rookie Kristaps Porzingis by noting: “He can shoot, he can make the right plays, he can defend, he’s a 7-footer that can shoot all the way out to the 3-point line. That’s rare. And block shots — that’s like a unicorn in this league.’’ Though unicorn describes the rare or nonexistent, its own metaphorical power makes it an increasingly common and useful word.

Unicorn implies a sense of wonderment often soaked in skepticism. It’s very likely that unicorn companies, so-named by Aileen Lee in 2013, are seriously overvalued, for instance. “Is your company a unicorn or a cockroach?’’ asked a recent headline in The Irish Times. Variations of a unicorn company include super-unicorn: a company valued at a whopping $100 billion. There are also decacorns (worth at least $10 billion), quinquagintacorns (worth at least $50 billion), centaurs (worth at least $100 million), and My Little Ponies (worth at least $10 million). Far less mythical is the beast named in a recent Forbes headline: “Could Theranos Go from Unicorn to Unicorpse?’’

The proliferation of companies in the unicorn club is considered a unicorn bubble. In The Week, James Pethokoukis referred to progressive techno-optimists as Unicorn Democrats. Another term that exists mainly but not entirely in the techie world is unicorn job, which is defined on University of Rochester assistant professor of computer science Philip Guo’s Web page as a job where “creative technically-minded people . . . can get paid to spend most of their time hacking on their own projects. Bad news: Unicorn jobs don’t exist, at least not for long.’’ Still, it’s hard to resist the possibility of professional perfection, and there’s a Unicorn Hunt Job Board for such seekers. A 2013 post on a message board shows that a unicorn job doesn’t have to be absolutely perfect, but the term can apply to any seemingly unobtainable position: “My unicorn job is any pharmacy related job at all so I can stay where I am living and not lose my apartment, boyfriend, old car. Sadly enough any job at all.’’

Anne Curzan, a professor of English at the University of Michigan and regular contributor to the Lingua Franca blog says, “I really like this new metaphorical use of unicorn as a modifier because it has that moment of surprise to it, where it reminds you that this thing we’re talking about — a job, a partner, etc. — is not just rare but it might not actually exist. A phrase like ‘dream job’ at least in theory means the same thing, but I think ‘dream’ is now common enough that it can mean that something is just really great, not necessarily that it is so magical that it may not exist.’’

The unicorn is an old myth that’s been around since the ancient Greeks, and unicorn has been an English word since the 1200s, spawning many related meanings along the way. The similarly horned, though real, narwhal has been called a unicorn — or sea-unicorn. Another type of horn informs the sense of unicorn as a cuckold. Since the 1700s, a carriage pulled by three horses, with one horse in front, has been called a unicorn, since it vaguely resembles the shape of a unicorn’s horned head. Monoceros — a synonym for unicorn — is the name of a constellation. There’s an old Scottish gold coin called a unicorn, and a South American bird also known as “the horned screamer’’ is called a unicorn-bird. Nature also includes the unicorn beetle, unicorn file-fish, unicorn hawk, unicorn horn-bill, and unicorn-moth.

Peter Sokolowski, editor-at-large for Merriam-Webster, notes that the etymology of unicorn is a factor in its success: “It seems to me that unicorn is useful in these contexts because it has become a kind of personification of unique — they both have that Latin root meaning ‘one’ that has come to mean ‘one and only’ or ‘rare and therefore valuable.’ There really isn’t a noun-for-adjective equivalent for words like excellent or expensive or attractive. This gives unicorn a powerful immediacy. And, of course, the word’s mythical past gives it resonance as a label in the hype culture of modern business.’’

Animals have an inherent metaphorical appeal, particularly if they’re rare. “Images based on objects that are at once familiar and exotic (like imaginary or African animals) get a lot of currency because of their vividness,’’ said lexicographer Orin Hargraves. “We have elephants in the room, 800-pound gorillas, zebra crossings, dinosaurs (to describe something obsolete), etc. Anything that lodges itself firmly in children’s minds is great fodder for metaphors in which some quality of the object is transferred.’’

Like unicorns themselves, word success can be hard to explain, and sometimes the experts have to make up a term to do so. David Barnhart, editor of the Barnhart Dictionary Companion, did just that: “The appeal of such terms often lies, I suspect, in the bizarre and fantastic imagery of the usual meaning of such terms. Sometimes I have called it linguistic sex appeal.’’ Barnhart provided a list of terms he believes also have such appeal, including binge-watch, blue-plate special, couch surfing, cyberbully, and fist bump. Linguistic sexiness is in the eye of the beholder, but it’s hard not to behold a little sexual energy in an animal with a phallic symbol protruding from its noggin.

This combination of linguistic sex appeal, animalistic metaphor, and mythological uniqueness makes unicorn a uniquely useful word. Even if most of the unicorn companies turn to unicorpses — hopefully, without impaling the economy on their horns — the word unicorn will continue to thrive and find new uses. The word’s stock might even go up as the unicorns fail — unicorns going bust would only reinforce the unreality of the entire concept.

Mark Peters is the author of the “Bull[expletive]: A Lexicon’’ from Three Rivers Press. Follow him on Twitter @wordlust.