There are various legends about how the Tigers got their nickname, one of which involves the orange stripes they wore on their black stockings. 1901 Tigers manager George Stallings took credit for the name; however, the name had appeared in newspapers before Stallings was manager. Another legend concerns a sportswriter equating the 1901 team's opening day victory with the ferocity of his alma mater, the Princeton Tigers.
In his 1998 book, "A Place for Summer: A Narrative History of Tiger Stadium", Richard Bak explains that the name originated from the Detroit Light Guard military unit, who were known as "The Tigers". They had played significant roles in certain Civil War battles and in the 1898 Spanish–American War.
The baseball team was still informally called both "Wolverines" and "Tigers" in the news. The earliest known use of the name "Tigers" in the media was in the Detroit Free Press on April 16, 1895. Upon entry into the majors, the club sought and received formal permission from the Light Guard to use its trademark. From that day forth, the team has been officially called the Tigers.
Edited from Wikipedia