Many do not know it, but at the time of GAMEBOY ADVANCE they sold cartridges with which you could watch episodes of some series on Nickelodeon, Nick Jr, Cartoon Network, Funimation, 4Kids and Disney Channel
I wouldn’t call this hidden, those things were everywhere towards the end of the gba’s lifespan. I had one for sonic x, dragon ball gt and a couple spongebob episodes, they were compressed to shit but it was still kinda cool at the time. It was rivaled by Videonow and juice box which also had the same idea.