It's really a debate as to what constitutes "cover."
I examined the rules on p. 192, but also sought out the rule for cover on p. 136 in the combat chapter.
Here is the section of the combat chapter explaining the rules on cover:-
If a target is in cover or partially covered by an object (such as a wall, door or fence), any attack that hits a covered location will instead hit the object. See Inanimate Objects on page 91. Cover affects both Ranged and Close Combat attacks.
The normal way of negating cover is to use the Choose Location Combat Manoeuvre to aim
at the visible parts of the target – or to strike through the cover if the weapon is capable of
penetrating it.
If the cover is total but the attacker knows his attack will penetrate the cover, he cannot use
Choose Location to target a specific location and must rely on randomly determining a location
(using 1D20) as per normal. In addition, the attacker suffers a –40% penalty to his attack. Note
that this only applies where the Attacker:
• Is 100% certain that the target is behind the cover.
• The cover is not so extensive as to permit the target to move freely behind it.
Thus, a target lurking behind a bush can still be attacked, even though the attacker might not be
able to see his quarry. However a target behind a lengthy hedgerow is completely obscured and
the attack will automatically fail.
Next, I looked at the Common Magic situational modifiers on p. 157:-
Partially obscured target –20%
Heavily obscured target –40%
Totally obscured target Automatic Failure
And finally, p. 192, with those rules regarding the range of sorcery spells.
Spells cast at a Range of Touch require the sorcerer to make a successful Unarmed attack on
a resisting target. To avoid touch spells a recipient must use Evade to dodge the attack, since
defending with a Combat Skill will still transfer the magical effect through the parrying weapon
or shield.
If Range is augmented beyond Touch then the caster automatically strikes his intended target,
unless the target is able to dive for cover or flee out of reach before the spell is completed.
So. Touch spells require an Unarmed attack as a followup to the Sorcery (Grimoire) roll to cast the spell. Touch spells are the only ones that can be Evaded - which is logical, if you have the image of Gronthus the Magnificent chasing after Ral, the Maiden of Partha, in the barn with his outstretched palms glowing.
Spells with an augmented Range greater than Touch automatically hit, if the cover is partial or heavy - the modifier affects the percentage change of casting the spell, not whether the spell strikes the target once cast.
Only if the cover is complete - behind a brick wall, for instance, or around a corner, and sufficient to completely cut off the line of sight between the caster and the target - will the spell automatically fail to connect.
So if the caster still sees the target's legs sticking out from under the horse, the spell still strikes the target and not the horse. Chances are, a spell like Wrack would affect that visible portion, giving the target a real dose of the hotfoots.
Of course the caster could sting the horse a little, causing it to spook and lash out at the target - but the caster would have to be casting Wrack at the horse.