What's with all the horse skulls?

Played through this game for the first time recently and had a wonderful time. Played it observantly and even discerned that the beacon would be used to cause the cleansing early on, but was wrong about why (I thought the prototype was the wrong Beacon, and the real one was in the Sun Coast in that cave you go through briefly).

Due to my attentiveness, though, I noticed an extremely common phenomenon throughout the game: human skeletons with the top halves of horse skulls. It's a little bit eerie, and way more common around areas where particularly fucked up stuff happened or is about to happen. For instance, if I'm recalling correctly, there are some in the cavern where you find one of Adila Dal'Varek's victims--the one who starved to death in a cage surrounded by food.

Does anyone know any lore behind these? Or perhaps symbolism? I've been scratching my head about it for two months now and haven't reached a conclusion.