It's an interesting idea you've got there, to seek answers in the past. (But don't put it on somepony unless he's really the blame, don't go picking random scapegoats, ok? )
Anyway, their 'past selves', as in 'in another life'? If so, probably some humans. I'd say some normal teenage girls with similar characteristics, but they're not exactly the same. Give them different names, appereances, a bit different character. For example, Twilight's past: make her nice and bookish, but socially awkward, because (for example!) past twilight never got the tutoring of the equivalent of Celestia, so she only cared for books and not for friendship.
But this is only an example, of course, ^^)