That strange Game Freak teaser wasn't for a Pokemon crossover at all, but is instead the studio's next 3DS eShop title. Soriti Horse is definitely a "surprise collaboration," but not of the franchise kind: it's a solitaire/horse racing game.
4gamer had the first report, helpfully translated by NeoGAF users. It explains that jockeys can improve their bond with horses by playing solitaire, then direct their horse around the track with the touch screen.
Play switches back and forth between the two phases several times in a race as players balance performance with solitaire-derived endurance. After the competition, you can share your speediest steeds via QR codes--which is a lot easier than that whole messy breeding process.
Soriti Horse will release on July 31 in Japan for 500 yen ($5). Game Freak's last title, Harmoknight, made it overseas, so with luck this one will make it abroad, too.