Hobbles prevent the pony from running away or kicking. However, in ponyplay hobbles are sometimes used just for sake of restraint.
In ponyplay many people will use bondage ankle cuffs clipped together as hobbles. This is certainly the easiest way to go especially if you already own a pair of ankle cuffs. However, I really like having a pair of dedicated hobbles that differentiate standard BDSM play from pony play. Hobbles meant for livestock usually have an integrated metal chain link that cannot be unclipped, and I like my hobbles for pony play to be styled similarly. For me, these hobbles reinforce the "you are now an animal" theme of ponyplay.
One common use of hobbles in ponyplay is for breeding (typically put on the mare to prevent kicking during breeding). For this purpose, I like Sinvention's breeding hobbles, which are heavy duty lockable hobbles with a livestock grade chain link, and like livestock hobbles, the chain link is not removable. This gives them an authentic look, which I personally love. Of course standard livestock hobbles work great and have an authentic look and feel to them. However you will hard pressed to find lockable livestock hobbles =)
Livestock hobbles can be made to be lockable by simply buckling the hobbles, feeding the excess through the buckle and retainer, then locking a padlock through the first available buckle hole. It's simple but it most certainly works, and your pony will not be taking them off without the key. The photo on the right illustrates this technique (although the photo shows a standard leather dog collar being locked in this manner, the principle is exactly the same with hobbles - though you will likely need a slightly larger lock for livestock hobbles).
As a human "forced" into ponyplay, I find the use of leather livestock hobbles with the heavy duty "livestock grade" metal chain link between them to be a powerful reminder of my animal status. Obviously this will not be the case for everyone, but I find genuine horse hobbles to be a serious turn on.