MTG: From the Vault Transform - Rules of Play says:
When they were introduced in Innistrad, transforming cards added a layer of intrigue and mystery to both the lore and the gameplay of Magic. Over the years, these cards have come to represent a diverse array of powerful changes on the battlefield. In celebration of this versatile mechanic, 15 favorite transforming cards have been assembled in From the Vault: Transform. Each card represents a different aspect of the power that comes with transformation, and each card has a story to tell.