Pictures & Explanation
Parrot Mouth in horses is a defect of the mouth that looks basically like an overbite or underbite where the upper and lower teeth don't touch.
Buck teeth is a slang term for parrot mouth. A buck-toothed horse is a horse with an overbite. Typically "buck teeth" are horses that extend over the lower lip.
By all accounts, parrot mouth in horses is genetic. That means you should not ever breed a horse that exhibits this type of condition. In fact, when showing "halter", a judge may ask to see a horse's bite.
If your horse has parrot mouth, the best thing to do is have him routinely seen by an equine dental specialist.
Treatment for any horse with parrot mouth includes regular checkups with an equine dentist familiar with treating the condition.