protecting a horse

specifically boosting the Stamina roll to prevent the target horse from dying of shock at the end of "Steed of Vengeance" spell.

If that Stamina roll is a Recovery roll, then "Rest of the Injured Brute" should help.

Looks like a SG or troupe decision to me. I would allow it. But the troupe might also require a specific CrAn spell, which mitigates the effects of returning from a Mutoed state only.

You could CrAn a horse and then not worry because the created animal will disappear. Not exactly what you want, but an alternative way to not have to worry about that death by shock.

You can always avoid death by shock simply by chopping its head off, or some other lethal maneuver.

Or reinvent the spell. The death from shock looks like a built in flaw from experimentation to me.

I've always liked the interpretation that a lot of the core rules 'does not quite work by base guidelines' is either imperfect creation from previous understandings of Magical Theory, or bonuses/penalties from experimentation side effects.

Really, there are a LOT of things in the core spells that don't quite line up with the rules for making your own spells.