If the party is outside, use bales of hay for decoration and extra seating. Make stirrups from construction paper and attach them to each chair. Cut out horse shoes and cowgirl hats from construction paper or poster board and hang them around the party room. Make a fence around the wall of the party room using rectangular pieces of cardboard as the planks. Make a mane decorating station. Set up a table with all sorts of ribbons, clips and other hair do dads for the guests to decorate their manes with.
Serve piles of straw. Pile up thinly cut French fries and serve your guests piles of straw. Have each child eat an apple the way a pony would eat it, no hands! Serve cowgirl food. Chili, hot dogs, sloppy Joes or grilled cheese. Serve horseshoe shaped cookies. Shape sugar cookie dough into horseshoe shapes and bake. Pre-made dough from the store makes this especially easy. Serve's mores. You can buy them pre-made, or make them to order for your guests.