Data miners have done it again! Some digging into the PC version of Sonic Mania was done and found an interesting, unused boss in the files. It looks like a callback to the mini boss from Act 1 of Flying Battery Zone in Sonic & Knuckles. It floats around the stage and you essentially play a game of rock, paper, scissors by hitting the button when it is on the correct counter to the tile on the right while avoiding its swinging arms. Here's a video of the full boss fight: