The girl who eats the porridge is a late arrival in her own story. For its first decades in print, the intruder at the bears' house had no name, no golden hair and no youth.
Robert Southey printed The Story of the Three Bears in his book The Doctor in 1837. His trespasser is a bad-tempered old woman. Eleanor Mure had written a verse version for her nephew in 1831, and hers is an angry old woman too. Folklorist Joseph Jacobs later suggested that the old woman may have replaced a fox, since one version, Scrapefoot, has a fox enter the house.
The change to a child came from the publisher Joseph Cundall. He made the visitor a little girl called Silver-hair, reasoning that there were already plenty of stories about old women. SurLaLune dates his book to 1849, while Mental Floss gives 1850.
Her hair then changed color. She was Silver-Locks in an 1858 collection. By the 1860s she was Golden Hair, in an 1864 book according to Mental Floss. The name Goldilocks appears with the tale in a letter of 1875, written in lowercase, as if describing hair. By about 1904 it was her settled name, and Flora Annie Steel's English Fairy Tales of 1918 helped spread it.
The word was waiting for her. Etymonline and Merriam-Webster both date goldilocks to the 1540s, as a name for a person with bright yellow hair. It joins Middle English goldy, "golden," with locks, tresses of hair. Merriam-Webster notes it was also used in plant names, translating the botanical names Chrysocoma and auricomus.
Since the mid-1960s the name has worked as an adjective for anything balanced between extremes, because Baby Bear's things were "just right." Merriam-Webster dates that sense to 1965. Astronomers speak of a Goldilocks zone, an orbit neither too hot nor too cold to support life. A name chosen for hair color ended up describing temperature.