According to Pausanias, the Arkadians claimed that Demeter’s unwilling union with Poseidon produced a daughter as well as the horse Arion. This daughter was called Despoina (the Mistress) in public: “This Mistress the Arcadians worship more than any other god, declaring that she is a daughter of Poseidon and Demeter. Mistress is her surname among the many, just as they surname Demeter's daughter by Zeus the Maid (Kore). But whereas the real name of the Maid is Persephone, as Homer and Pamphos before him say in their poems, the real name of the Mistress I am afraid to write to the uninitiated” (Pausanias, Description of Greece 8.37.9).
Jennifer Larson says in Ancient Greek cults: A Guide that Despoina seems to have been an older Arkadian goddess who had a strong affinity with Artemis. I didn’t include Despoina in my story since it would have complicated things too much. It just wasn’t believable that my version of Demeter would have given away the baby that was not a horse.
Yeah, but it would’ve been interesting and maybe Desponia stayed behind to look after her brother.