Animal glue is an organic colloid of protein derivation used as an adhesive, sizing and coating, compo, and for colloidal applications in industry which is derived primarily from collagenous material present in animal hide or from the extraction of collagen present in animal bones, primarily cattle or derived from recycled gelatin.
These protein colloid glues are formed through hydrolysis of the collagen from skins, bones, tendons, and other tissues, similar to gelatin. The word collagen itself derives from Greek κόλλα kolla, meaning 'glue'. These proteins form a molecular bond with the glued object.
Stereotypically, the animal in question is a horse, and horses that are put down are often said to have been "sent to the glue factory". However, other animals are also used, including rabbits and fish.