ReefkprZ nailed it right on the head. There
are some corals you can keep with ponies, but not many. In general it's better to keep seahorses in a smaller aquarium. Ponies like less flow than most reef tanks, and because of their very deliberate feeding habits you have to choose carefully the other tankmates to have around your sea horses.
LPS corals are strictly a no-no. They, like anemones, are far too likely to sting the horses. You can get away with a few soft corals, but the difficulty in keeping them is almost prohibitive. The cooler water, added to the reduced water flow, makes keeping them a constant battle.
If you really want ponies I'd suggest (humbly, 'cause I'm new!
) another tank entirely. The Good news: you won't really need some unbelievably expensive light for the seahorse tank -- since there's no corals, there's no great need for metal halides. More good news: Since these do not require photosynthesis, you can consider gorgonians for a pony pen. Gorgonians are also great as hitching posts for the horses. Just make sure they get enough flow to keep algae from growing on them. If you do decide on a new tank, try for one that has a good amount of vertical swimming room. Sea horses tend to use the whole water column.
Finally, if you like there are also plenty of good tankmates that you can add if you want a little more than just seahorses in the display. Pick animals that are shy, reclusive, and picky feeders. It's a great opportuity to display both your wonderful sea horses as well as a fish or three that you might not be able to keep with the more agressive animals in your reef tank. For my part, I chose a royal gramma, red firefish, lyretail anthias, and a single mandarin. As I'm sure you know, just make sure there's plenty of live rock and preferably pod piles or a refugium to keep the pod population stocked. Also, pipefish are excellent to keep with sea horses. They feed in the same manner almost.
Sorry this post took so long!!
As you can see I was pretty worried about doing the right thing when I decided I wanted a seahorse tank. If you do a google search, you can find a number of different seahorse forums with people that will be more than willing to help you get started. Good luck!
