Despite being a ground mount, Tishan's Fluffy Transport from the Raj'Dur Plateaus Lockbox doesn't allow you to use a ground mount appearance over it. I'm not sure if this is intentional or not, but considering it lets you use flying mount appearances over it, I find it bizarre that it won't allow ground mount overlays as well.
Yeah same, my ground mount skins won't show up on it at all, just shows the default bear skin, but flying mounts will cover it up. Weird stuff.
This was already reported in this thread. But since it is only a visual bug (an annoying one at that) it takes low priority for being fixed.