What mouse is compatible with iMac 21.5 inch late 2012?

Any USB mouse will work with the basic left/right/scroll buttons using the universal drivers.

For fancy mice with extra buttons or Bluetooth mice check the box/manufacturer for MacOS support.

I am using a plain old wired mouse with mine. I also have a Magic Mouse 1 but I don't use it. I think your question should be rephrased and say which mouse is not compatible. Probably pretty much and kind that has a USB wire or Bluetooth capability would work.

i used a varitity of both bluetooth and usb mice with my old imac 21" 2011 never came across one which didn't work, though wireless mice that are not bluetooth but comes with some usb dongle will likely require a mac driver to work

I have a logitech wireless mouse that comes with a USB dongle that I have used with Macs as old as 2006 models. It does not require a dongle but it does, of course, require the dongle be plugged into a USB port.