Unlike with your pony, the savage attack is usually much more than a single lunge and bite. If that is correct, then there is hope that the behavior can be corrected and that this pony can be a useful and safe companion animal. At this point, I would recommend a trainer who is experienced and competent with rehabilitating this sort of case. It would be great if you could find someone who specifically appreciates pony personalities. The trick now is to get the pony to feel completely safe with people and to understand that it is specifically and only biting that will result in punishment.
Most successful trainers use a gentle, reassuring handling technique.
They use ample positive reinforcement for good behavior and very well-timed, judicious punishment for the biting. After the well-timed punishment, they immediately relax and proceed. It typically takes a couple of weeks or less for the pony to return to normal, dependably safe behavior. I have seen horses and ponies with very severe problems get the message within a few minutes of interaction with an expert. There sometimes is an advantage to beginning the retraining in an environment that is new to the animal, and working with one or more expert handlers who are relatively consistent in their methods.