Slats, appearing from 1917-1924, born in Dublin Zoo
Dietz said he decided to use a lion as the company’s mascot as a tribute to his alma mater, Columbia University, whose athletic team nickname is The Lions.
The first lion, called Slats, was used for the original Goldwyn Pictures design and for the first MGM version. He didn’t actually roar, preferring to people watch.
Jackie, 1928-1956, the second lion to be used in the logo
Jackie’s roar was recorded for use at the beginning of MGM talking movies. A sound stage was built around his cage to make the recording.
In addition to appearing in the MGM logo, Jackie appeared in more than one hundred films (all black and white films from 1928-1956, including the Tarzan movies that starred Johnny Weissmuller).
Coffee was one of two lions that were used for two-strip Technicolor test logos on early MGM colour productions.
Coffee, 1932-1935 two-strip Technicolor
MGM began producing full three-strip Technicolor films in 1934, and used Tanner for all Technicolor films from 1934-1956.
Tanner, 1934-1956 full three-strip Technicolor films
The sixth lion, officially named George, was introduced in 1956.
Leo, the seventh lion, is MGM’s longest-lived, having appeared on most MGM films since 1957. He was also the youngest of all the lions at the time MGM filmed his roar (hence the smaller mane).
Hitchcock “directing” Leo in 1958, photo by Clarence Sinclair Bull
In 1965, in attempt to update its image, MGM recruited Lippincott to create a more contemporary logo. The result was known as “The Stylized Lion,” and it appeared at the front of three films in the 1960s: Grand Prix (1966), 2001: A Space Odyssey (1968), and The Subject Was Roses (1968).
The stylized lion, 1966-1968
Leo was reinstated afterwards, but a refined version of Lippincott’s mark is in use today for MGM Resorts International.
More detail about each lion on Wikipedia.