This article contains speculated information, and therefore it may be inaccurate and non-canon.
The name of the episode she appears, "Love Triangle", is a joke with the fact that she is triangular shaped.
Her real name is confirmed to be Goldie Goldenglow on a recent Nickelodeon press release. Other prior names (i.e. "Goldie Goldengirl" or "Goldgirl") are mistranslations.
She is portrayed by Tara Strong who also voices Timmy and Poof
Previously it was stated that fairies are unable to have babies due to what Cosmo did when he was a baby. If Goldie IS a fairy, then this brings up the question of how she could exist. Possible reasons for her existence include:
She is only half-fairy, thus making her existence as a younger baby possible.
The ban on fairy babies was lifted after Poof was born, but nothing has been stated in the show as saying so.
She is not a fairy at all, making the rule not apply to her.