Have you ever wanted to find fish with your Game Boy? Now you can with the Pocket Sonar! Released only in Japan in 1998, Bandai's "Gyogun Tanchiki: Pocket Sonar", aka Fish Finder: Pocket Sonar, hooked up to your Game Boy and contained a fish sonar, fish encyclopedia, and even a fishing mini-game! Check out this video from The Gaming Historian!
https://youtu.be/5mHSHmk_UU4
Speechless. Really. And in 1998. And only compatible with original GB. But the GB is essentially a mini computer just like every console, so why not.