Hollow Mockeries have the appearance of the corpse of a horse, which has been whipped around the head and neck, so that it is covered in blood and strips of flesh, and much of the skull is revealed. They are usually festooned with bells and bright bows, as if they were to be presented as a gift.
Most scholars agree that they may be created when a person dies in a way which caricatures the purpose of their entire life. The idealist who builds so many prisons in the name of freedom that he is eventually hung as a tyrant; the woman who, desperate for a child to nurture, steals another's baby and then starts a house fire while cooking for it, killing the baby and herself; the hunter who seeks food, freedom, and solitude in the forest, and then starves to death, caught in their own trap and with no one to call for help. The spirit of such a person knows no rest as the morbid irony gnaws at them, and so the corpse releases melancholic vapours which can coalesce into this spiteful creature. That at least is the most commonly-accepted theory of their origin. Refer to the entry on goats for another.
Since their insides are filled only with vapour, they are best slain with an arrow or spear. A wound from these weapons is likely to cause the creature to pop, or to fly away wildly, like a balloon which has been inflated and then released without being tied. In the latter case the spear or arrow will be lost, but this is a small price to pay.
The same effect might be achieved with a stab from a dagger. However the traveller is not recommended to get close to the creature, for fear of its sharp hooves, the wounds from which resist healing. Still less is it advisable to fight the creature with a club. Having no internal organs to damage, it is unlikely to have much to fear from that weapon.
(partly based, with permission, on this post by Phoenix Talion).