The Mane Event
Invitations: Get a picture of a horse off the internet. Print the picture multiple times (for however many guests your having). Take a piece of white paper and cut it into the shape of a speech bubble (again one for each picture). Cut out the horse and tape or glue the speech bubble to it. Write inside the speech bubble the date, time, and place of the party. Tell them to come in jeans or riding pants.
"Mane" Event: Try to find a place that offers trail rides and take your friends on a trail ride (with your parents permission). If you have your own horse give each guest a mini riding lesson. Show your guests how to tack, untack, lead, and groom a horse. Let them have a go each at this. Roundpenning is fun too. Have a few unpainted model horses and paint ready. Let each guest paint a horse.
Food and Drinks: Get horse themed snacks (ie baby carrots, apple slices, sugar cubes, peppermints, etc). Serve in "food troughs". Just make tiny fake food troughs out of tin foil or paper. serve water to drink. Horses don't drink Coke after all!
Games: Have a jumping course. Make jumps out of household items. If you can use real jumps. Time each person over the jumps. The person with the fastest time and/or least number of poles knocked over wins.
For Sleepovers: Rent a few horse movies
(I recommend Danny, The Black Stallion, Second Chances, Secretariat, or National Velvet). Karaoke is always fun. If you like scary stuff take turns telling ghost stories and/or get and Ouija board.
Party Favors: Let the guests take home the model horse they painted earlier. If you take pictures during the "Mane Event" part of the party give copies to each guest.