Bill Cipher used stones turned from ponies in making his throne in the start of the fight and mentioned that "pasting all this bootleg merchandise together sure took a lotta glue. Good thing your friends make their own".
This is referring to the history that animal corpses have been used and refined as glue in ancient times (back to about 2000 BC) when glue was used as adhesives, binders and finishings. (Nowadays animal glue as adhesives are more restricted as synthetic adhesives are easier to produce and are more popular. Gelatin is extracted from animal bodies as jelly for dessert ingredients. Animal hides are dried and mixed as pet food. Some companies, such as those in Canada, still produce animal, hide and hoof glues from horses. Recently, animal glue has been replaced by other adhesives and plastics, but remains popular for restoration. Still, animal glues will darken with age and shrink as they dry, giving them the potential to harm wood, paper, or works of art. Too much handling and too many changes in temperature or humidity could cause further harm.)
en.wikipedia.org
Bill Cipher is truly one very weird and evil lord.