First of all.. The duallayerclothes.sdata has nothing to do with pets or wings. It is only for the costume models.
Pets and wings are pretty easy to understand when you are just taking a further look into your client...
Pets:
Pretty easy.. models are starting in Monster.sdata on ID 4001. The 4001 represents the pet model ID "1" in Item.sdata. So as on the screen below... 1 = Eagle, 2 = Fox, and so on... You can just enhance the models by using the further id's 4001 - 4100 ? Or 255? Not sure about how many models can be added. There you would have to try out.
Wings:
A little more complicated but still easy to get through it when you would take yourself some time to figure it out. The models are specificted in the Wing.mon, but ALSO in the Monster.sdata. Check the values in each file.
You can also adjust more ID's of wings (121 5 - 121 255) in your Item.sdata.. Use the search function here for it.
Anyway.. i actually already said too much 'bout something which is easy to understand on your own when you just put some effort on getting through it..