Samsung will bring a foldable smartphone to Mobile World Congress according to the latest rumor coming out of Korea, though it won't be on display at their press event if the rumor is accurate, and will instead be shown behind closed doors, likely to members of the press who have previously scheduled appointments. If Samsung were to be planning to show such a phone off in secret it's entirely possible that they could indeed have a foldable smartphone in the works with a prototype that they want to parade around, as they wouldn't have to worry about the public's opinions of a device that wasn't in a ready state.
Samsung is rumored to start production of their foldable smartphone within a year and a half from now, and if that timing is close to what Samsung has in mind and is what they're shooting for, then there is likely a lot of work to be done still before moving into the final production phase. In addition to alleging that the phone will be shown to people behind closed doors at the event, the rumor also states that Samsung is looking for feedback on the product from people that are in the industry. If that is Samsung's reasoning for showing off the phone then they will likely want to take that feedback and use it to make any adjustments to the phone where needed, which could include the phone's outer design, the software experience, features, hardware, and more.
Since these are just rumors, there is no guarantee that Samsung will actually showcase the device at the Mobile World Congress, and even if they do it's quite possible that they won't let any of the information go public and may require anyone who sees it to keep from detailing what they saw. Rumors of a Samsung foldable mobile device have been floating around for a while. Late last year a rumor surfaced alleging that Samsung would end up releasing their first foldable smartphone in the third or fourth quarter of 2017. Even more recently, a leaked render that is allegedly of Samsung's folding phone prototype showed up on the web as of earlier this month. Samsung has also been granted a number of patents on folding phone designs, so regardless of what stage they're at with the product, it seems clear that they're at least tossing around ideas.