desk chair for a girls room.  Looking for odd chair.  Theme of room is horses.
Maybe a chair that is shaped like a saddle.  Chair cannot be big in width 18-20"

Here's the perfect choice for your 'horse themed' room.  Check out
this unique saddle stool that you can find at: 
http://www.claybornesamishfurniture.com/html/hoof_stool.html
The stool is available with and without hooves and you can get it
without the saddle horn as well.  Look around in that area - there are
several options.