Why is there a cow on the Elmer's glue bottle?

Answer 1

It isn't a cow, it's a bull named Elmer, husband of Borden's Elsie the cow.

Some glues are made from rendered cows hooves (as are Gelatin products like Jel-O), leading some people to joke about the cow on the Elmer's bottle being representative of its ingredients.

In an episode of The Simpsons, vegetarian Lisa hallucinated that the cow on her glue bottle accused her with: "You won't eat my meat, but you glue with my feet."

Answer 2

The cow on the Elmer's glue bottle is a symbol of the product's main ingredient, which is animal-based glue made from collagen extracted from cow hides. The cow represents the source of the glue used in the product.