This is a pulley cover for the rear Y axis pulley on the Anycubic i3 Mega (I guess it's a gear attached to a stepper motor, not exactly a pulley, but that's what everyone is calling it so we'll go with that)
I decided I wanted some pulley covers, and I found one that someone else made for the front, but not too many for the rear. I didn't like the look of the ones I did find for the rear, and the one I tried sat higher than I liked, and also didn't stay flat well because it was only held down on one side. This is a bit lower profile, and attached with two screws to help keep it lying flat.
Check out my other Anycubic i3 Mega accessories!