most gardeners will say to hot compost it, to remove pathogens etc, before putting it in the garden. also horse crap has a lot of weed seeds so composting will help, although some seeds survive the heat generated by fermenting. The community garden I am involved in spread a lot of old (several years old) horse manure all over the plot, but then the flood came and went and a lot got washed away.
I second the post saying to compost it. Several years ago I put two pickup truck loads of raw horse manure out of a barn on my garden and tilled it in. Certainly helped boost my vegetables, but I ended up with with a carpet of morning glories too (I never had morning glories there before the manure), and I've been battling them ever since. Apparently horses love morning glories, and passing through their digestive tract encourages the seed to germinate tremendously!
I spread a BUNCH one year next to a horse barn since there was PLENTY of it.
It wasn't composted enough.
LOTS of nitrogen. Everything started out great, but within a couple of weeks I had the best looking oat patch you've ever seen that had completely choked out EVERYTHING.