I think ponies based on user names comes very close to crossing the creepy line, but as long as it is just a picture based on a name that seems innocent enough to me. Now if people started to consider them characters and do comics or stories about them, that's when I think it enters original character do not steal territory.
On an unrelated note, here are a couple of wallpapers I found that might make great desktops:
First up is Canterlot - evening by Crappyunicorn, who also did a sunset version of the same picture earlier which you can see
here. It'd be awesome if anyone here knows a program or tool that changes your desktop based on the time of day, because I think it'd be pretty cool to have this set as your wallpaper and have it automatically change the version when it gets to night or day.
Next is Pinkie at the Gala by ShelltoonTV. It's a 1080p screenshot from the end of the gala song, but since there isn't a clear 1080p version of the episode I think it's a vector trace from the 720p version. He says on his page that he is going to do the other 5 main characters so I'll post them here when he does.