[2023] This dark season is the perfect time for us to bring you the saga of... Invisible Killer Ponies! Look into comments to learn more!

These creatures come from Pathfinder: Kingmaker. In early development, the ponies in the game were invisible, as they had no model. This is where the bugs came into play. Because of one, the "Summon Monster" spell only summoned ponies. And because of another, the summoned "monsters" attacked the summoner.
As a result, in the alpha version of Kingmaker it was possible to summon a small herd of invisible ponies that chased the character around the map.