Some glue comes from horses. It has become a less popular manufacturing process as synthetic options have become available. Elmer's, one of the largest suppliers of glue, has stated that none of its glue is derived from animals.
Horses are not the only animals that were traditionally used to make glue. Pigs and cattle have been used to create glue as well. The collagen in the animals' bones and connective tissues is processed and turned into a sticky gelatin substance.
Animals have been used to create glue for thousands of years. The earliest known instance dates back over 8,000 years on artifacts found at Nahal Hemar Cave in Israel.