Image source: Fitbit.

Who doesn't like rooting for the underdog? Sometimes companies that have the odds stacked against them prove to be compelling investing opportunities, since the market will often price in that apparent disadvantage. But sometimes it's not that clear-cut, which is why we're adding these names to our watchlists.

Evan Niu, CFA: It might be hard to call Fitbit (NYSE:FIT) an underdog, considering the company is easily the dominant player in the wearables and fitness tracker markets, but hear me out. Sure, if you look at unit volumes as of right now, Fitbit is the top dog, leading the market by a healthy margin. It enjoyed an estimated 30% market share in 2015, twice that of its next closest competitor, according to IDC. However, if you consider the market's trajectory going forward and how the rankings will inevitably shift in the quarters and years ahead, we get a different picture altogether.

Specifically, smartwatches are coming for fitness trackers in a big way, and investors can expect aggressive categorical cannibalization here. This is easily the biggest threat to Fitbit's dominance right now. Additionally, if you look at the rate of technological innovation and advancement that's playing out right now in smartwatches compared to fitness trackers, Fitbit is definitely an underdog on this front. The company continues to add new features to its lineup, but the major mobile platform operators have armies of third-party developers who will innovate relentlessly to improve smartwatch performance, adding new functionalities that probably no one is even considering right now.

As far as the wearables market in the long term is concerned, I believe that Fitbit is an underdog to retain dominance. That being said, shares are not terribly expensive, trading at 22 times earnings, and Fitbit is profitable both on a GAAP basis and a non-GAAP basis. It can still build a sustainable business even with smartwatch cannibalization and high abandonment rates. There is a price at which I'd consider investing, but it's not there yet. I'm keeping an eye on this underdog in disguise.

Todd Campbell: It can be downright impossible to carve away market share from deeply entrenched players, but that's not stopping Depomed (NASDAQ:ASRT) from trying, and so far succeeding, in the multibillion-dollar market for pain medicine.

Last year, Depomed acquired Nucynta from Johnson & Johnson (NYSE:JNJ) for $1 billion. At the time, Nucynta had a sales run rate of a couple hundred million dollars and less than 1% market share in pain treatment. However, a tripling of Nucynta's sales force and a price increase that brought its cost in line with the top-selling Oxycontin led to prescription volume surging 20.7% year over year in December and fourth-quarter sales of $68 million, up 55% from a year ago.

Nucynta's revenue increase has Depomed forecasting that its revenue will grow to between $485 million and $525 million this year, up from $342 million in 2015. It also has Depomed thinking that Nucynta's peak annual sales potential could be as high as $1 billion.

Because Depomed is growing sales rapidly, is already profitable, and is arguably cheap with a forward P/E ratio of about 10, this is one underdog I like long term.

Tyler Crowe: Any company that is trying to upend the century-long hold that oil has had on the transportation market should be considered an underdog. Clean Energy Fuels (NASDAQ:CLNE)is one such company that's trying to solve possibly the greatest challenge to alternative fuels replacing gasoline and diesel: building out fueling infrastructure.

Consider this for a moment: Today, there are about 2,900 fueling stations for natural gas and LNG as a road transportation fuel. That may sound like a decent number, but compare that to the more than 120,000 gasoline and diesel stations nationwide. Without a comparable network of fueling stations, customers aren't going to make the switch from gasoline.

Clean Energy Fuels has been picking away at this by going after some of the easier targets such as fleet vehicles and heavier-duty service vehicles that don't travel far from a central hub like waste collection vehicles and public transit. As these vehicles start to switch, it should give the company a greater revenue base from which to build out a roadside fueling network.

It may be a long way off from building a fleet of fueling stations that make it a compelling alternative to oil-based fuels, but cheap, abundant natural gas suggests that it could be possible some day. As long as that condition holds, it's worth watching Clean Energy Fuels' progress. 

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.