While you wouldn't want to apply horse manure directly to plants (it'll burn them), letting it compost first works great! You could make a huge compost pile out of it by taking turns layering the manure (nitrogen) with carbon such as wood shavings or straw, etc. It'll take some months depending on the zone you live in and which season you begin your project.
I also have good luck using a thick layer on top of an existing bed (or a new one, for that matter) at the end of summer after harvest and letting it sit for several months. The earthworms do their work and it's ready for planting like magic come spring.