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Seriously. Try lassoing a horse, and see what happens. (All speedy hins, cavaranmasters, and domesticated animals like cows will attack you.)
It's important especially since people just leave their horses everywhere.
Horses should have the 'Neutral' tag, not 'commoner' which causes the effect you are seeing (Indeed all those cows etc should also be neutral).
This might just be a mistake by myself, whereby many have been assigned the wrong faction, though I suspect it may be that NwN assigns faction commoner when they are set free. I hope it is the former, in which case we can (gradually) fix it.
Thanks for letting us know.
(Moving this to bug forum)