Let's hope that the gas is easy to get out.
The easiest way to extract the gas in your pony's pouch is to grab the seahorse. Bring him to the water surface without, obviously. taking him out of the water. Let his tail wrap around your pinky and hold his head between your index and middle fingers. Use your other hand to split open the pony's pouch. Use your thumb and push up from the bottom of the seahorse's pouch. Slowly move your thumb up until the top of the pouch. Lots of tiny bubbles should come out. Repeat this to make sure that you got all the bubbles out. Let go of the seahorse and see if he sinks to the bottom. If he sinks, you got most if not all the bubbles out. If he continues to float, grab him again and keep on pushing bubbles out of his pouch.
Some times the bubbles are too big to come out of a seahorse's pouch, in which you have to take extra measures. Trap him in your hand the way I described above. Take a flat-ended paperclip in your other hand and gently place it in pouch. Do not poke him too hard because you'll risk damaging him. Push up on the pouch with your thumb while the paperclip is in his pouch. Lots of bubbles should come out. Move your thumb up his pouch again to make sure that you got all the bubbles out.
When you capture the seahorse in your hand, he will curl up into a ball to protect his pouch. Push his head aside slowly and eventually he will stretch out his tail, at which point you should stick your pinky out for him to wrap it around.
After the whole procedure, your seahorse will probably stay curled up in a ball for a few minutes. Don't be alarmed. It is completely normal.