The name Buttercup is a girl's name of English origin. Buttercup and is often added to lists like Fictional Princess Names and discussed in our forums with posts like "Baby a Day".
From the experts:
Though most Buttercups are of the bovine persuasian, this humble flower name was given to the lovely princess in "The Princess Bride." If Buttercup still feels too lowly for you, you might consider Clover, Daffodil, or Daisy.