After failing many times to get the cheat provided to work, I had to take things into my own hands. Here's how you do it~
Start by identifying your Ponymon ID number here: http://ponymondawndusk.wikia.com/wiki/Game_shark_cheat_codes#comm-14706
Then find the corresponding Pokémon here: http://www.supercheats.com/gameboyadvance/pokemon-firered/13900/Wild-Any-Pokemon-100-Level/ (this is not the cheat you want to use!)
When you find out which Pokémon lines up with your wanted Pony, search their name here : http://www.supercheats.com/gameboyadvance/pokemon-firered/13758/Any-Pokemon-you-want-on-the-gr/
Copy this as your catch code: D2BF38B5 7E300C38 8343300B ADEF57CA
And then put the cheat line of the pokemon in the gameshark cheats below your catch cheat. Then you should have no problems finding your pony! Mind you the game is unfinished and many of the later on pony sprites are not filled in, meaning your discord will just be a Mew and your Luna a dragonite.
Ponymon Dusk/Dawn by Flutteryay Music Yakity Sax Credit picture source: http://www.deviantart.com/art/My-Little-Ponymon-Yay-Wallpaper-292719600
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the catch any ponymon cheat has never worked for me, BUT I found a way around that, if you want...