The Detroit Lions were founded in 1930. The first name of the team was the Portsmouth Spartans. They moved to Detroit in 1935, and the team was renamed the Detroit Lions.
Baseball had the Tigers,so the football franchise selected Lions.
From the NFL Hall of Fame: "DETROIT LIONS - The Lions name was chosen by George A. Richards, the Detroit radio executive who purchased the Portsmouth Spartans and moved the team to Detroit in 1934. "The lion is monarch of the jungle," a team spokesperson said, "and we hope to be the monarch of the league."
In 1932 they were the Portsmouth Spartans.
William Clay Ford
In 1929, the Portsmouth Spartans were formed, in 1934 the team requested a name change and became the Detroit Lions, following the jungle cat theme started by the Detroit Tigers.