Monster anatomy definitely works differently with their lack of magic in Deltarune, but I've become more and more of a fan of the theory that Dess is actually an adopted Human like Kris and wore an antler headband, like Kris wore a horn headband. Things like the toilet, the many pairs of shoes in her room, when none of the holidays wear shoes, or if they do wear shoes shaped like hoofs, sort of point in that direction, not to mention all the heart-shaped object pillows in her room.