Very good question ! (and i dont know the answer)
I would say IMO it would be nice for the prefix to be of that of the seller etc as it was their choice to put x to y etc but i imagine the name would be totally down to the new owner for them to call whatever they like? Would the 'breeder' on the passport be classed as the person who covered/AI'd etc or the person who owns the mare at foaling?
sorry to throw another question into the mix!
i think I know the second question,
Im pretty sure, the breeder is the the person that owns the mare when foaling. so with that in mind I would presume that person picks the prefix.
A Prefix has to be registered with the Central Prefix Office for a breed Society to accept it otherwise you could have ten Prefixes the same all over the Country.