If, during your declare attackers step, Ursine Monstrosity is tapped, is affected by a spell or ability that says it can't attack, or hasn't been under your control continuously since your turn began (and doesn't have haste), then it doesn't attack
It's super flexible, as you can use it when your own horde swings in, or when another player goes in on the offensive (they don't even have to be sending everything/anything at you.) However, while I can see this card working pretty well in any blue/white/green tokens deck, it's obviously best with an MTG commander that cares about birds
Granted, unlike those starter decks, this game doesnt include additional booster packs or any foil cards
It only takes one mill-10 trigger with the monarch to make this card worth its in a graveyard deck
The Blue-Red Limited Archetype: Instants/Sorceries & Adventures Blue and red mana combine to make "The Sorcerer's Apprentice," a spell-heavy archetype that also makes good use of adventures