If the monster selected by "Dark Jeroid" is removed from the field by "Dimensionhole", or flipped face-down, or returned to the hand or Deck, or sent to the Graveyard, etc., the effect of "Dark Jeroid" is reset and the selected monster no longer has a lowered ATK.
If "Dark Jeroid" is the only monster on the field when it is Summoned, it reduces its own ATK.
The ATK cannot go below zero.