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

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.

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.