"Buffalo" are indigenous to South Asia (Water Buffalo) and Africa (Cape Buffalo), while Bison are found in North America and parts of Europe. While this is true, the mistake is not made by this series but by the popular culture. American bison were and are referred to as "buffalo" by the populace, and using the term "buffalo" was not a mistake by the writers. They used the word as everyone does.
Clyde is wearing a collar when he first starts to follow Charlie.