2019-07-12 : 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.

2019-07-12 : 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.