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.

If a creature the defending player controls can’t block Prized Unicorn, it can block any attacking creature, or not block at all.

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.