Seriously... I bet you could make them out of a variety of materials depending on:
1. How many horses are there?
2. What are your concerns regarding runnoff, capacity, smell, ability to clean the bin, expected lifetime of the bin, durability, etc.
The USDA might have some info. Also, there's the American Society of Agricultural Engineers,
Are you looking for a supplier or a design reference?
If you want to design it, I would think that a standard retaining wall would do the trick. At least, that's what I've normally seen in stables. Imagineer