Autumn Fey is a general term for a categorization of fey creatures of Otherworld based on ancient Elven Lore.
“And Esseldan, firstborn of the Elves,
came then to the great Forest called Coire.
So great were his travels that the stars had changed,
and another season had passed between one land and the other.
In the autumn there he awoke the creatures of the forest,
and he bid them speak with him.
Thus were the Sylvani woken beneath the stars of the autumn season,
and their domain was the forest, and all that was in it.”
-Part of The Song of Esseldan Recited and Translated by Ingolion Liritano.
The Autumn fey are those creatures originally from Coire in Otherworld, ruled over by the Wood Elves. Including: Firbolg, Satyrs, and others.