Everything old is new again. Back in the Windows 95 days, there were a handful of freeware games that played with no window and entirely by displaying a changing icon in the system tray. I remember having executables for clones of Pong, Space Invaders, Tetris, and chess that played this way with the icon as their only 'screen'.
I had a little sheep which walked on top of windows, and occassionally fell off them.
When I switched to Linux, I was astonished that sheep.exe worked on Wine, too. This was ca 2000.
There’s a web version we can embed on our personal websites now!
Broken link. Fixed: https://github.com/Adrianotiger/desktopPet