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.

Ambulanceroid: You can Special Summon “Dark Jeroid” with “Ambulanceroid’s” effect because it has “roid” in its card name.

Lone Wolf: If your opponent Summons "Dark Jeroid" and reduces your "Monk Fighter" ATK by 800 points, then later you activate "Lone Wolf" targeting the "Monk Fighter", it does not restore the ATK of "Monk Fighter".

Rescueroid: You can activate "Rescueroid's" effect when "Dark Jeroid" or "Elemental Hero Necroid Shaman" is destroyed by battle and sent to the Graveyard because they have "roid" in their card names

Shrink: If a monster whose ATK is reduced by "Dark Jeroid" is affected by "Shrink", first calculate half that monster's original ATK, then subtract 800 for "Dark Jeroid's" effect; this becomes the monster's current ATK.