Where we used to live the local travellers had horses and I saw them (deliberately) putting a heavy coloured cob of 15hh+ to a tiny Welsh sec A mare (we were trying to ride our young just broken in horses down the adjacent bridleway at the time :-OO). So it's not always an accident!
In this case I would steer well clear, if it were me. I wouldnt want to acquire a horse from someone who knows so little about them that this could arise, it would make me wonder what sort of care they had in other respects (eg worming, vet care). And the bills if this pregnancy went wrong could be huge, quite apart from the trauma for you.