Ninten-Switch.com reports that the trademark was recently discovered. It's been tweeted out by a Japanese trademark bot.
— 商標速報bot (@trademark_bot) October 6, 2017
The trademark was filed on September 15, and included this Game Boy image.
Here is how the trademark listing appears on website Japan Platform for Patent Information.
[Image: J-Plat Pat]
It doesn't specify the name on the trademark, but it covers a variety of "home video game console" programs as well as "programs for smartphones", "smartphone cases" and "smartphone covers", as well as "key holders", "necklaces" and "watches". Some of the trademarks could be to protect the "Nintendo Classic Mini Game Boy" brand or they could simply be to protect the Game Boy brand. Full stop.
Back in 2005, Nintendo released a smaller version of the Game Boy Advance called the Game Boy Micro.
Gizmodo Japan is one of many Japanese sites wondering if this means a Nintendo Classic Mini Game Boy is coming and pointed out (via US Gamer) that this past winter, Nintendo Europe also filed for a Nintendo 64 trademark. With the success of the NES Mini and the SNES Mini, it would be surprising if more Minis were not in the works.
Kotaku reached out to Nintendo prior to publication, but had not heard back at time of writing.