CrAn spell level help

I want to make a ritual spell that creates a horse that has thicker than normal skin. Essentially combining wizards mount and doublet of impenetrable silk.

Since this is a CrAn ritual I see no need for any muto requisites. What would you add for the unnaturally thick skin? or is this still covered by the base create a mammal? I dont think it's such a major change to warrent base 50 magical animal.

Subsequent versions would make smarter/faster/more agile versions how would i base these?

Well, it is no longer a natural animal, so... base 50 it should be.

You could create such a beast temporarily, sure, with a muto requisite and a one magnitude bump or so.

Your best bet is enchanting a saddle with a constant effect muto, I think.

Add one magnitude for each of your additions, I'd say.
By making the animal smarter, do you mean smart for a horse, or inteligent like a familiar? Because that's a lot harder.