In a previous blog post, Horse Eating Slower, a feeder was shown to help a horse eat slower.
Another option is a hay net: BusyHorse Hay Net
Directions for making the Cataloochee Free Choice Pasture Hay Feeder
Here's an innovation: Pasture Pal Feeder
Nose-It Feeder Toy:
Soaking Hay with Slow Down® Hay Feeder Soaker