In the 19th century, who was known as a "Peeler"?

A London police officer. So named for Sir Robert Peel who established the police force in 1829. He is also where the term Bobby comes from! Sir Robert completely reformed the police system with a professional force. His plan was not initially popular and many feared threats to civil liberties, but the metropolitan police became a legal force in 1829 and ended up being a net positive for safety.

https://www.britannica.com/topic/stipendiary-police