A: Goldin, Heritage auctions, eBay auth guarantee for high end
Finally, this card was the centerpiece of one of the most dominant deck archetypes of all time
The most likely reasons are if it's sacrificed, if it's legendary and another legendary permanent with the same name is controlled by the same player, if it's a planeswalker and another planeswalker with the same subtype is controlled by the same player, if it's a creature with toughness 0 or less, or if it's an Aura that's either unattached or attached to something illegal
Planning ahead can give you some great combinations
(And yes, multicolor creatures count for purposes of this question.) White Giant White Merfolk White Treefolk Blue Elemental Black Elf Black Goblin Black Faerie Green Kithkin White Treefolk, Black Faeries and Green Kithkin