Refers to the lip of an animal, especially in a more colloquial context.
El belfo del perro se movía mientras ladraba, mostrando su entusiasmo y energía.
The dog's lip moved as it barked, showing its enthusiasm and energy.
El belfo del caballo se movía con gracia mientras trotaba por el prado.
The horse's lip moved gracefully as it trotted through the meadow.