C Common Frightfur March X TRAP Counter When an opponent's Spell/Trap Card, or monster effect, is activated that targets a "Frightfur" monster(s) you control: Negate the activation, and if you do, destroy that card, then you can send 1 of the targeted "Frightfur" monsters to the Graveyard, and if you do, Special Summon 1 Level 8 or higher "Frightfur" Fusion Monster from your Extra Deck
How good is the backside
With Miirym, generating Treasures allows you to cast expensive dragon creatures and copy them while keeping enough mana open for removal and counter spells
But the best ability is the +1/+1 counter transfer to all your other legendary creatures, and that alone justifies putting it in a legendary-matters deck
Cards with protection from these colors also cant be the targets of these spells
That could make all the difference in how much play he sees main deck as opposed to being in sideboards