What did Princess Fiona turn into at night?

Answer 1

After Shrek sings "Big Bright Beautiful World (Reprise)" to Fiona, she runs off stage. Just then, the Fairytale Creatures run on stage, pushing her back on stage, with her back to the audience. Except that it isn't her; it's a body-double wearing a matching wedding dress. Meanwhile, off stage, human Fiona does a costume and makeup quick change into ogre Fiona. Later, during the commotion after the "surprise" wedding guest confronts Lord Farquaad, Fiona trades places with the body double. When she turns around, she is revealed to be an ogre.

Answer 2

She was actually a princess to begin with. A spell was cast on her so that she would turn into an ogre every night. When the spell was broken, she could basically choose if she wanted to stay a human or become an ogre. She chose to become an ogre and marry Shrek.

Answer 3

Ogre