If a creature the defending player controls can’t block Prized Unicorn for any reason (such as being tapped), then it doesn’t block Prized Unicorn. If there’s a cost associated with having a creature block Prized Unicorn, the defending player isn’t forced to pay that cost, so it doesn’t have to block.

If more than one Prized Unicorn is attacking, creatures that could block must block one Prized Unicorn, but they don’t all have to block the same Prized Unicorn.