Many years ago (my dd is 16 now) she was into horses too..her 5th birthday. She has a summer birthday so that worked for here in Minnesota. We had a bigger crew (10-12 if I remember..cousins and neighbor kids), and I paid a pony ride place who came to our home (suburban street) with a dwarf horse little dude, some petting zoo animals (a pair of bunny, pygmee goat, 2 kinds of chickens and?? something else?), plus the gals teen daughter who did GREAT face painting. So we had the kids in smaller groups, but all close to each other, and all were busy and not sitting in long lines in the heat.
My Mom and I had gone to the party and dollar stores and got cheap straw cowboy hats, sheriff badges and red bandana's for each child. We also made a simple..super simple..pattern for a vest (3 pieces...back panel and 2 front ones...we made them in general sizes to accomodate the age of children coming), and got some thin fakey vinyl type "cowhide" stuff at the fabric store. Each vest required a running stich line up each side and across each shoulder. No finishing on seams, etc as it was this vinyl stuff. So each child had a neck bandana, hat, vest with sheriff badge and looked the part!
We also got a few hay bales to decorate our area for pictures and to welcome the kids...no missing which house it was, and we just put them behind the shed and used again for Fall decorating and then to spread around plants for winter insulating.
Then we had gotten other bandana's in a variety of colors and made those the goodie bags..with a few candies, a few toy horses and such for them to take home. During the party we took pics of all the kids on the horse..and another of each one with the birthday girl next to the horse. When we sent THANK YOU's we sent copies of both of those pics to each child.
Good luck and have fun!