Such a buffalo (part of the Asian buffalo family, not related to the American bison) once lived in the Philippines. On the island of Cebu, the buffalo formed an isolated population whose members shrank in size by about two-thirds over time, resulting in an animal shorter than the largest domestic dogs and weighing about 350 lbs. The bones of the only known example were found fifty years ago in a phosphate mine by engineer Michael Armas, who kept them without thinking much of them until he showed them to specialists in 1995. Estimated at ten to twenty thousand years old, the remains are now the basis for a formally described species, Bubalus cebuensis. The buffalo is an important example of "island dwarfism," a phenomenon in which island populations develop smaller size compared to their counterparts in mainland environments. (In an amusing example of the vagaries of evolution, the feet of B. cebuensis did not shrink as much as the rest of the animal, so it has disproportionately large feet.)
The concept of island dwarfism has been most famously debated in the case of the proposed hominid species, Homo floresiensis, the "hobbit" from the Indonesian island of Flores.