Why are female swans called pen?

The female swan is called a pen due to an old British law. According to the law, in the 12th century, all unmarked swans in the country belonged to the Queen. These unmarked swans were referred to as "the Queen's game." Over time, this term was corrupted to "Queen's pen," and eventually, "pen." So, the term "pen" is used to refer to a female swan due to this historical British law.