One thing you can say is that items (so also leathers) which are written in grey are useless for crafting. They're just supposed to be sold for some peanuts. So Zhevra leather could disappear from the loot list.
This is a complete list of leathers which can be skinned. It includes many exotic things which could be added with a lower drop % just to have a source for these at all. I agree that this is a list which would be way too long so this is not a perfect solution.
But much more important are
these "Hides" for upping one's leatherworking skill level. They are needed for many leatherworking craftings and unlike the normal leathers cannot be crafted upwards. So I think we could use at least these hides on the drop list.