Japanese frogs are proliferating across Asia. The good news is, they're not an invasive species, nor are they real.

"Tabi Kaeru" ("Travel Frog") became the No. 1 downloaded smartphone app in China for almost two weeks after its debut, and it is still hovering at the top of the charts in China, Hong Kong, Taiwan, Singapore, Malaysia and Japan. The idea for the game came from Mayuko Uemura, a 26-year-old employee of developer Hit-Point Inc. who has never written a line of computer code.

The game's objective is simple: Pack a lunch, maybe a tent, plus a few other trip-friendly trinkets for your virtual amphibian and wait for him to come back from his travels with pictures and gifts. If the premise of gathering food and knickknacks while waiting for animals to show up sounds familiar, that's because it is: Nagoya-based Hit-Point is behind the cat-collecting game "Neko Atsume" and came up with the latest hit. Both titles share DNA with Tamagotchi, Bandai Namco Holdings Inc.'s handheld virtual pet toy that became a global fad in the 1990s and early 2000s.