Considering the History of Formor as given they make the most sense given their aquatic nature. When the Fae Mothers came to power all the other elder races where either killed or forced into the Sea aka the Formor.
I think the exact quote from Ghost Stories is:
“Why the Fomor?” I asked. “I mean, I barely know who they are. Why are they all over Chicago now? Who are they?”
“Once, they were the enemies of my people, Winter and Summer alike,” she said, lifting her chin as her emerald eyes grew distant. “We banished them to the sea. Now they are the exiles of myth and legend, the outcasts of the gods and demons of every land bordering the sea. Defeated giants, fallen gods, dark reflections of beings of light. They are many races and none, joined together beneath the banner of the Fomor in a common cause.”
Which comes after this bit from Bob:
Bob’s skull rested in a container that had been custom mounted on the Road Runner’s dash—a wooden frame set on a plate that made the skull wobble back and forth like a bobblehead doll. “Lot of interbreeding there,” Bob said. “Back in the old, old, old days. Before the Sidhe Wars.”
I lifted my eyebrows. “I haven’t heard much about it.”
“Crazy stuff,” Bob said with tremendous enthusiasm. “Even before my time, but I’ve heard all kinds of stories. The Daoine Sidhe, the Tuatha, the Fomor, the Tylwyth Teg, the Shen. Epic alliances, epic betrayals, epic battles, epic weddings, epic sex—”
Which (of course) is a reference to the short story Curses
which introduce one of those groups as an extant wyldfae court.
Hope this helps.
Richard