Well Arkay is an Aedra but he was originally a mortal raised to Godhood by Mara.
"Lies from a previous age." - http://www.imperial-library.info/obscure_text/nu-hatta.shtml
Compare the Story of http://www.imperial-library.info/dfbooks/b005_arkay.shtml with Tu'whacca. It's a parable about finding purpose in mortality.
Tu'whacca (Tricky God): Yokudan god of souls. Tu'whacca, before the creation of the world, was the god of Nobody Really Cares. When Tall Papa undertook the creation of the Walkabout, Tu'whacca found a purpose; he became the caretaker of the Far Shores, and continues to help Redguards find their way into the afterlife. His cult is sometimes associated with Arkay in the more cosmopolitan regions of Hammerfell. - http://www.imperial-library.info/mwbooks/varietiesoffaith.shtml
She's a wayward solar daughter cast from the heavens for consorting with illicit spectra. In another words she wasn't originally a Daedra Prince.
Neither were the Daedra. To be exact, there were no Daedra before there were Aedra and no Aedra before Daedra. Before the creation of Mundus, there were other ways to group the et'Ada though. Usually between Anu (Solar) and Padomay (Lunar, Lorkhan).
It's commonly overlooked because the Aedra generally conform too the lighter side while the Daedra hold the darker side. There are exceptions though. It's not difficult to imagine Solar et'Ada that never participated in the creation of Mundus. Just as it's not difficult to imagine an Lunar et'Ada that did participate in creation, Lorkhan.
With the creation of Mundus allot of the Lunar et'Ada changed. Mehrunes used to be a Leaper-Demon, according to Faith in the Empire Sheogorath came into being (nevermind the crap from SI) and Hermaus Mora was born from ideas that were never used in creation. Seems to me this would also be the point where Meridia went from being a Solar et'Ada with a somewhat questionable moral to full blown Daedra.
So to answer your qeustion. It depends on how you define Aedra.