A report on Yahoo!'s Japanese-language finance site mentions Nintendo's plans for a new handheld device and even gives it a codename, but it isn't clear if this is a 3DS successor or a piece of hardware attached to its next console.
This unit, described in the report as a "next-gen handheld game device," is codenamed MH. It could be a stand-alone unit, a unit that cooperates with the NX but is sold separately, or the unit that controls the console. Or something else altogether.
In October, the Wall Street Journal reported that the NX would include "at least one mobile unit that could either be used in conjunction with the console or taken on the road for separate use." And a patent application published at the end of last year pointed to a new handheld, which featured an elliptical touchscreen embedded in a traditional twin-handled console controller's chassis. (That became the basis for an elaborate hoax purporting to show the controller/handheld back in March.)
Nintendo didn't comment on its plans then and probably won't now. Still, Polygon has reached out to a Nintendo of America representative to ask what's going on. Overall, Nintendo has said NX is launching in March and it will not reveal, tease or discuss the console at E3 in three weeks. Its only product at the expo will be The Legend of Zelda game coming to Wii U and NX in 2017.