It is unusual to find a yearling semi-feral pony that bites humans. Most are timid and would rather go away than engage in confrontation. Biting is an instinctive and natural behaviour in all horses, one of the manufacturer's setting if you like. Nevertheless, it's not one you see in many young horses other than as a playful, investigative behaviour. The mouth is the equivalent of the opposing thumb and very useful to slow things down, manipulate and retaliate.
With Taita you begin to wonder if there is more to it than that; something genetic. I know of another foal with the same father who was also quite aggressive and it could also be that her mother was an alpha type mare. Beyond nature there is nurture and Taita could have copied her mother if her mother engaged in the same sort of behaviour particularly around humans.
I am great believer in cause and effective-ness and I think Tiata's behaviour was brought out first by a fear of being eaten (we know that she went through the drift, was branded, went through the sales and then was put in a crush twice -once for her microchip and once for a headcollar to be forced on. She was also wormed.) Once a prey animal can no longer run away from a predator, it can only bite the predator, kick it or squash it. Secondly it was activated by a very strong desire to take and then protect her food. The behaviour manifested itself when there was food around. The most important factor though is that the behaviour has been successful in making people go away. I also suspect that she is sore in her neck (having probably been swung) and she may even have gastric ulcers from the stress of having been handled roughly and moved so many times with a sudden change of diet each time. This would be enough to make any pony grumpy.
I have no doubt whatsoever that given time and training this behaviour will go away. In her particular case, I would never connect food with her training and would simply give her her feed and move away. In time I would make sure that I could stand by her and even touch her while she was eating but I would not use clicker training at all.
It would be important to make sure that the biting behaviour was never, ever, successful in making people go away or making people give her food. This isn't going to be easy for the owner in a shared field surrounded by people who will want to feed the pony over the fence. Whilst I would let her bite the feather duster so that she learns that biting it never makes it go away, I would react to any attempt to bite a human with some meaningful body language - underneath she is still timid and all it takes is a little kiss kiss noise and arm movements like an angry blackbird, nothing more. I would never turn my back on her or not have her in my peripheral vision whilst working with her or moving about in her field; once she is being led I wouldn't want her behind me. Once Taita knows that the behaviour doesn't work and most importantly that she has no need to use it, I am sure she will be fine. Yesterday proved that she is very trainable and that she does like the right sort of touch and will accept a headcollar readily.