Well I found LilPip one that I liked from a thread on Ponychan [link] but i haven't been able to find the others. Maybe I'm just not looking hard enough, but I don't know. If i cant find the others, I'll probably just make them.
So, the first thing you need to make sure you do is download DesktopPonies, which you do from HERE. Desktop Ponies lets you have ponies run around your desktop (if it isn't obvious enough) so, what you want to do if you want Calamity on your screen is download the .zip file, then just drag the "Calamity DP" folder into the Desktop Ponies folder. Thats all you really need to do.