Since update R41 May 1st 2017, you can now use Autumn Leafi Leaves in two crafting recipes for potions:
These crafting recipes will accept all types of Leafi Leaves as a crafting ingredient. If you want to use Autumn Leafi Leaves for the recipes, just choose the according crafting recipe, and click on the blue arrows on the Leafi Leaf icon to browse through the options until the icon for Autumn Leafi Leaves is shown.
Like other animal material Leafi Leaves cannot be put into your quickbar and cannot be placed into the world. They also cannot be displayed on Stone Wall Shelves, Placemats, Hidden Temple Altars, in Wood Planters nor in Flower Pots.
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 Autumn Leafies mainly in Forests, spawning during (ingame-)daytimes around or on all types of Autumnwood Leaves, including Fallen 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 attack them or try to tame them. Additionally to their leaves they can also drop/give Autumnwood Flowers, blocks of Autumnwood leaves, Bones, Sinews, Globs of Goo and/or rarely Leather.
If you want to harvest Autumn Leafi Leaves from your Pet Autumn Leafies, you will have to feed them their exact favorite Food type. Otherwise they will provide you with a much less interesting harvest.
For example; if Autumn Leafi Pets prefer to eat ordinary Sandwiches, then a Turnip Sandwich will lead to a reduced pet-harvest. Or if it prefers to eat Horned Melons, then even feeding it "better" Food like Melon Pie will still result in a pet-harvest with less items.