In Greek mythology Atalanta the virgin huntress was abandoned by her father in the wild and was cared for by a bear:
Lycurgus had sons, Ancaeus, Epochus, Amphidamas, and Iasus, by Cleophyle or Eurynome. And Amphidamas had a son Melanion and a daughter Antimache, whom Eurystheus married. And Iasus had a daughter Atalanta by Clymene, daughter of Minyas. This Atalanta was exposed by her father, because he desired male children; and a she bear came often and gave her suck, till hunters found her and brought her up among themselves.
Source: Apollodorus. Apollodorus, The Library, with an English Translation by Sir James George Frazer, F.B.A., F.R.S. in 2 Volumes. Cambridge, MA, Harvard University Press; London, William Heinemann Ltd. 1921.
Enkidu's story is certainly older, however, I would assert that Enkidu was not abandoned as a child - in fact, was never a child - and thus does not quite fit the feral child motif. In any case, even if Atalanta's story isn't the earliest, it's certainly unique in that it is about a female child and not a male one.