I thought it was more simple than that. I always assumed "done" meant that something which had been prophesised in one of the many books, had come to pass. In the case of the horse - that was meant to be given to Errand by Garion to establish a bond between them? But "Done" appears quite a few times in the series and it's always when some which was pre-ordained has come to pass.
The voice which says "done" is the voice in Garion's head (in our world, he'd be given medication!
), which is the voice of the 'good' Fate.
My 2c anyway.