Thanks for the replies! Exodus, if you do use the horse assassin do let us know how it works out. >
Thanee, you are correct that is impossible to use Spellcraft to identify a still, silent spell.
There's no rule that says a Still, Silent spell does NOT provoke an AoO. However, flatfooted opponents do not get to make AoOs, and I'd rule that a person unaware of the horse's nature (in other words, a person that has no reason to threaten the horse) would not get to take an AoO on the spellcasting horse. It's the same reason why a friendly fighter doesn't take an AoO when his wizard buddy casts a spell.
Darklone, Detect Magic would not detect Wildshape.
From SRD on Detect Magic:
"Aura Strength: An aura’s power depends on a spell’s functioning spell level or an item’s caster level. If an aura falls into more than one category, detect magic indicates the stronger of the two."
A wildshaped druid is not an item, and is not under the effect of a spell. Until the druid lays on a buff spell there just aren't any auras to detect.
If one of the party members was a druid or ranger then I'd let them make Know: Nature checks to see if they notice that the horse doesn't quite act like a horse--call it an unconscious "Wild Empathy" check that takes a day or two.
Of course, if the assassin really *is* a horse (an awakened horse druid/rogue/assassin/shadowdancer) then there really is nothing to detect. It's a horse, man. An evil, deadly horse, but still a horse.
-z