One of the most highly watched phenomena in the past few years has been the ascendancy of the unicorn. Whether 2015 will go down in the record books as the peak of this tend remains an open question, however.

These private companies, many of them startups, are valued at $1 billion or more--sometimes much more. Yet as rare as their mythical namesake suggests, the actual list of such companies has swollen to include 142 names, according to venture capital research company CB Insights. Nearly half of the companies joined the list in 2015 alone.

But it could be that 2016 is the year that so-called unicorns lose their horns, or become "uni-corpses," as some financial experts like to quip.

All kidding aside, several larger economic factors substantiate this threat. Unicorns were created in the aftermath of the financial crisis, when the low interest rate environment prompted investments in riskier assets, such as the stock of privately held companies. However, the Federal Reserve increased its benchmark interest rate in mid-December, which is likely to have a direct impact on fundraising and force down the high valuations of many of these late-stage private companies, venture capitalists and economists say.

Even in the weeks before the Fed's move, highly valued private companies faced other pressures as prominent mutual fund companies, such as Fidelity Investments, bid down the value of their holdings, potentially over concerns that they had become too bloated. Then, as more unicorns consider going public in the next year, investors and markets could change their tune regarding their embrace of these companies. 

Despite the challenges that lie ahead--or perhaps because of them--some unicorns are sure to prove more interesting to watch than others. From the most outgoing to the most imperiled, here's a look at five companies to track in the New Year.

1. Uber

The car share company, which operates in 63 countries and 300 cities, is currently the most valuable privately held startup, worth a staggering $52 billion, and its valuation could notch even higher as it is reportedly seeking a new round of financing, reportedly worth $1 billion. By some estimates, its valuation could soar to more than $70 billion, which would make it more valuable than either General Motors or the Ford Motor Company. At some point, however, the company and its co-founder Travis Kalanick will need to exit, either through a 2016 IPO or some other event. If it's the former, investors will have the last word on whether its valuation is sustainable.

2. Airbnb

Similar to Uber, Airbnb boasts an enormous valuation. Following a November funding round of $100 million, the company's valuation shot up to $25 billion. Whether or not the public markets--through an IPO--would support that figure may prove the least of its worries. Prominent regulatory challenges and voter initiatives in a number of big cities question the legality of the San Francisco-based company's operations.

3. Palantir Technologies

There is plenty of buzz around this super secretive big-data crunching tech company, which has been credited with using its sophisticated pattern detection software to bring down Ponzi scheme operator Bernie Madoff and Al Qaeda terrorist Osama Bin Laden. In December Palantir was reportedly close to sealing nearly $700 million in new financing, putting its valuation at $20 billion. Some security analysts, such as Avivah Litan of Gartner, the research firm, say some businesses are questioning aspects of the company's services, particularly the length of time Palantir requires for its engagements. If that's the case, 2016 could be the year the bloom comes off the rose of one of the most highly valued private startups in the U.S.

4. Snapchat

The disappearing message app's $16 billion valuation may seem downright puny compared with its other top unicorn peers, but startup founders everywhere admire founders Evan Spiegel and Bobby Murphy's chutzpah when they swatted away Facebook's low-ball $3 billion offer in 2013. Of course, Facebook is now experimenting with its own disappearing message app, which is one indication that the competitive landscape for the Venice, California, company has ramped up. And this fall, it was one of several companies to have the value of its private shares bid down 25 percent by Fidelity Investments. Still, Spiegel has reportedly said the company has plans for an IPO soon. So 2016 may be the year.

5. Zenefits

The online health care benefits company is also part of the cadre of unicorns whose private share price has been tapered by mutual fund investors. More specifically, Fidelity slashed the value of its Zenefits shares by 48 percent in the third quarter of 2015. That doesn't necessarily harbinger the worst for the company. Its hub and spoke model, whereby it offers free service like payroll and accounting in addition to its core, brokered health care options, has captured the interest of tens of thousands of small business customers, the company reports. And health care is likely to remain a top concern, as the Affordable Care Act mandate for small businesses kicks into gear in 2016.