Marauding Maulhorn checks whether you control a creature named Advocate of the Beast as attacking creatures are declared. At that time, if you don’t, Marauding Maulhorn attacks if able.

Marauding Maulhorn can attack if you control a creature named Advocate of the Beast. It just isn’t forced to.

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.