Horse manure is especially high in macro and micro nutrients, unlike ruminants who's multiple stomachs remove more 'goodies' from whatever they eat.
Don't add masses of manure just because you can get as much as you want: I did that and levels of salt, phosphorus and potassium climbed high
enough for the soil technician to suggest I lay off the superphosphate!
Nothing like it to get a compost heap going.
As others have said, unless you're a gun composter, you'll get weeds and inevitably they'll be extra tough ones you don't already have.
Can you also get old straw/hay/sawdust from the stable? Best mulch ever...