If you defeat a Boss Dragon, you will change into a new form. Early in the game, you cannot change form at will. But, as the game progresses, you will find devices that allow you to select which form you take. This is useful, as you may need to transform into a specific form in order to proceed through certain sections, later in the game.

Simply land on a transformation device to change form. To change form again, jump up and land on the device.

Below are the forms you will take during your adventure: