If there's a cost associated with having Ursine Monstrosity attack the chosen player, you aren't forced to pay that cost, so it doesn't have to attack that player in that case either
Besides a reliable flicker effect, Thassa also makes for a well-sized attacking creature as long as you can get enough blue permanents in play
In short, it's power and toughness are equal to the most expensive artifact you've got in play, and you get some mana on each of your upkeeps that never goes away
Slightly less rare than these, but still incredibly valuable, are the Shinies added as part of the Shining Revelry expansion - alternately variants of cards like Charizard, Lucario and more besides
The slower your draw, the more damage Black Vise does
Its not 100% clear to me what exactly that sixth horn is for, but this text makes it clear that its bad, and I dont want to be one of the people learning firsthand