I'm looking at some MuAn effects, and noticed that Steed of Vengeance (ArM 119) has a rather nasty side-effect built in.
The Muto Animal guidelines don't mention this side effect. The base 15 seems reasonable for the physical enhancements to hooves, teeth, and appearance as a freebie.
They don't mention how this effect is able to grant +5 Attack and damage either, so I'm leaning toward this was a funny side-effect of experimentation, which is also why the Stam vs 9+ is included.
I'm a fan of that spell, except the part where the creature must made a stamina roll of 9+ or die when the effect ends. It seems to be a meta-mechanic to discourage a player from using an otherwise great spell (well a great spell if you're happy to kill or conjure a horse).
To look at it another way - my Magus takes the Steed of Vengeance as written, drops the Duration to Momentary, and you get a level 25 spell which can momentarily kill a horse with a resisted Stam 9+ check. At PeCo 25, that's perfect for the base 20 + Touch kill a horse.
And since it is suggested that you look at Animal effects for Corpus suggestions, you could extrapolate it to a Human of Vengeance as well. Yup. Manifold implications.
If that is an issue in your saga, you can always drop the Stamina check. The +5 to damage is just that well, the creature is supposed to become awesome, so it should be awesome. I do not see a problem with that. You are basically riding a demon-looking horse (now that is a nasty side effect when you meet some Templars in the road) so a +5 damage seems good. It is a legacy spell in any case, so feel free to modify it if it does not fit your saga's tastes.
Maybe you could change the roll to a Brave roll instead, and the animal becomes edgy and intractable for Duration +1 (so Moon duration if Sun, Diameter if Instant) if he fails it? less radical but can still stop a knight charging you quite well. Normal for a level 25 spell in any case.
Quite possibly (probably? my bet, anyway) it's a legacy element in the description from earlier editions off the game/spell. 3rd ed (and certainly previously) still had random elements in the spell descriptions that echoed back to a certain classic RPG and contemporaries that shall not be named. I bet if you searched in old spell lists from those other RPG's, you'd find a spell very much like that, or close enough to explain it.
My (least) favorite is Whispers Through the Black Gate, where an InMe effect on a corpse, inexplicably, allows the "voice" of the spirit to be heard by all who are nearby. Without a MuMe for them, or a CrIm, or anything that might explain that particular effect, other than "hey, it's magic!".
I'm pondering a MuCo/An effect to transform into a giant Kaiju, or to turn an animal into giant monster versions of themselves; and I didn't want that spell cited to show a stam check was needed afterward. Agree it is probably just legacy description, and I'm sure we'll be more logical. Not that it is often logical to turn into a giant monster, but every mage needs a hobbie.
I do not see a problem with that. You are basically riding a demon-looking horse (now that is a nasty side effect when you meet some Templars in the road) so a +5 damage seems good. It is a legacy spell in any case, so feel free to modify it if it does not fit your saga's tastes.