Most ponies were bred where the terrain is harsh and the grass is sparse and therefore it is in their DNA to be thrifty. This is why they are SO EASY TO FOUNDER!
When feeding hay to ponies, it should always just be good quality grass hay. There is absolutely no need for a healthy pony to have alfalfa or clover. The key to making sure your pony stays healthy in all seasons is to put your hands on them a lot. A fluffy winter coat can cover up what condition your pony is really in. (http://www.eaglefernequine.com/wp-content/uploads/2014/10/Sep_2014_Equine-BCS.pdf)
Feed just enough hay to keep your pony in the proper body condition. Too fat is just as bad as too thin. Exercise is also extremely important for ponies. They really should not be exclusively stall kept. If one must be stalled, investing in a slow feeder for hay would certainly be a good idea.
ALWAYS PROVIDE MINERALS EITHER LOOSE (preferably) OR IN BLOCKS FOR YOUR EQUINES.
In the summer time, ideally your pony should have to work for its food. A pasture with very slow growing or sparse grass would be great for it. Some ponies must be kept entirely on a dry lot.
Here is an article that gives good advice on feeding ponies.