If, during your declare attackers step, Marauding Maulhorn is tapped, is affected by a spell or ability that says it can’t attack, or hasn’t been under your control continuously since the turn began (and doesn’t have haste), then it doesn’t attack. If there’s a cost associated with having Marauding Maulhorn attack, you aren’t forced to pay that cost, so it doesn’t have to attack in that case either.