The Female Butterfly is one of the 24 Golden Bugs in Twilight Princess. Its male counterpart is the Male Butterfly. This bug can be traded to Agitha for a Rupee prize.
The Female Butterfly is located around the east part of Hyrule Field to the east of Castle Town (Wii), or in the west part of Hyrule Field to the west of Castle Town (HD).
This bug is situated on top of a ledge, meaning Link needs to utilize either his Clawshots or his Boomerang to obtain it.
Female Butterfly's location in Hyrule Field
Male and Female Butterfly's locations on the Wii map