The costume will someday be available for all the pets. But so far there are only 5 pets that can be headless:
This is where the News was released: http://www.marapets.com/news.php?page=50
When your pet survives, he will get a charisma status point. So you want to bring all your pets to her except those listed.
Visit this link to see the Decapitating Fairy!