Hi:
 
Some horses have a sweet tooth and will pull the frames and eat the honey.
I've seen some act much like bears - get stung, run off, come back later,
eat some more wax and honey.
 
A horse is a really large animal.  Bee stings are generally of little
consequence (in terms of risk to the horse) as long as the horse can run
away.  There was an incidence a couple of years ago in Utah when two horses
tied to some junk for a period of several hours.  The horses kicked over
some nearby hives and could not get away.  The horses were put down by a
Vet (we don't know whether the bee stings would have killed them).
 
A rider on a horse that is stung by a bee or wasp is at risk - a friend of
mine was threatened with a lawsuit when an outfitter's horse stepped near a
wasp nest in the ground.  The horse raced off, throwing the rider, who
broke a leg.
 
As per the benefits of packages versus nucs - don't forget that a package
is less likely to carry disease spores (such as foul brood that might be in
the wax or honey) and the lack of brood will cause a brood break which may
be helpful in controlling mites.  Hopefully, all nucs are well medicated
and inspected.  No doubt that a nuc will get a faster start.
 
Cheers
Jerry J. Bromenshenk
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http://www.umt.edu/biology/bees