All creatures able to block Prized Unicorn do so.
10/1/2009 Prized Unicorn doesn’t give a creature the ability to block it. It just forces those creatures that are already able to block it to do so. For example, it can’t force a creature that’s tapped or affected by a spell or ability that says it can’t block to block it. If there’s a cost associated with having a creature block, its controller isn’t forced to pay that cost, so the creature doesn’t have to block Prized Unicorn in that case either.
10/1/2009 If a creature the defending player controls can’t block Prized Unicorn, it can block any attacking creature, or not block at all.
10/1/2009 If two Prized Unicorns are attacking, each creature the defending player controls can block either one of them. (The defending player chooses.) A creature that can block multiple creatures, like Palace Guard, must block both of them.