There will be a chance that your bike will not be equipped to fit a certain pannier. If this happens to be the case but you have already fallen in love with one, you still may have hope. It may be as easy as simply installing a specialized rack on your bike. But, it all depends on the design of your bike.
To make it easier on yourself, you should just find a pannier or any type of bag, quite frankly, that will mount correctly onto your bike.