Shapeshifter

Oracle Text

As Shapeshifter enters the battlefield, choose a number between 0 and 7.

At the beginning of your upkeep, you may choose a number between 0 and 7.

Shapeshifter's power is equal to the last chosen number and its toughness is equal to 7 minus that number.

Card Rulings

8/1/2008 The ability that sets its power and toughness is a characteristic-defining ability. This means it applies before any other effects that modify power and/or toughness.
8/1/2008 The ability that lets you choose a new number is a triggered ability that triggers only once each upkeep.