There are three variants of Autumn Leafies that are of slightly different colors, but they will nonetheless all drop the same kind of spotted Autumn Leafi Leaves. You can find them in Forests spawning during (ingame-)daytimes around or on Moccha Autumnwood Leaves that often cover the ground under Autumnwood-trees. Rarely they can also be found around other trees like Ashenwood.
Autumn Leafies are peaceful surface-creatures, but will defend themselves if you try to tame them or even attack them. They do not seem to have any special attacks themselves. Additionally to their leaves they can also drop/give Autumnwood Flowers, blocks of Autumnwood leaves, Bones,
As of yet this material is of no use for any crafting-recipes and does not "unlock" any crafting-recipes either.