BUFFER, Slang, applying to people or animals. OED
Buffer 3, meaning a fellow: usually expressing a slight degree of contempt. In print from 1749.
Buffer 1, a dog, or a dog-like person. From 17th c.
Buffer 2, a foolish fellow; Scottish and dialect, 19th c.

Also other meanings, not slang.

Agreed, buffer as slang no longer used in U. S. A., but appears in some 18-19c. materials.