Dinosaurs! Feathered and pink and teal. Poppy, Sal’s best companion, is one. Muna, the Pretty Egg Girl, is another version, or at least related.
One was found in the Cathedral being experimented upon, but they started showing up in more force in Season 13, near Shepherd’s End. Suspected to come from another world. Seem to be various subtypes, large, small, and sapient.