Itylus

In Greek mythology, Itylus, or Itylos, was the son/daughter of Aedon and King Zethus of Thebes.

Aedon accidentally killed him/her and was stricken with grief and guilt. In pity, the gods turned her into a nightingale, which cries with sadness every night. Alternatively, Aedon attempted to kill the son of her rival, Niobe, also her sister-in-law, and accidentally killed her own son/daughter instead and thus, the gods again changed her into a nightingale.