In the Dresden Files universe, there are many kinds of shapeshifter, not all of them human in original nature. While many of these shapeshifters are limited (often to a single animal form like werewolves or weresharks), a rarer few don't have such restrictions on the forms they can take. They may still be limited - only able to disguise themselves as another person, or only able to take on the forms of true, natural animals - but some are limited only by their imagination, and their skill with their ability.
Some of these freeform shapeshifters are naturally born with the potential - a "minor talent" who's somewhat less minor than most - or have put in years of hard work learning how to do it, much like those who learn to take on a Wereform. Others are tied more directly with mythology; perhaps you've been infused with a drop from Ceridwen's cauldron, or you're a descendant of a trickster like Loki or Coyote.
Musts
Shapeshifters must have a high concept that in some way indicates their supernatural ability.
Shapeshifters must take the Physical Transformation condition.
Shapeshifters must take one of the available shapeshifting powers.