There are many commercially available round and square bale feeders that are raised off the ground with a solid base (some even have a roof to protect from the rain) with small openings where the horses can reach in and access hay. They are marketed to prevent a lot of hay waste from horses dropping hay on the ground and then standing on it. Check these out at your local farm supply store or online equipment dealer.
If you use round bales, it is recommended to place a wooden pallet on the ground inside the feeder and then place the hay bale on top of the pallet. This is to raise the bale off the ground and out of the mud.
