Leo the Lion Spoiler

Leo the Lion is the mascot for the Hollywood film studio Metro-Goldwyn-Mayer and one of its predecessors, Goldwyn Pictures, featured in the studio's production logo, which was created by the Paramount Studios art director Lionel S. Reiss.[1]
Since 1924 (when the studio was formed by the merger of Samuel Goldwyn's studio with Marcus Loew's Metro Pictures and Louis B. Mayer's company), there have been around five different lions used for the MGM logo, including Tanner[2], and Leo, the current (and fifth) lion. Tanner was used on all Technicolor films and MGM cartoons (including the Tom and Jerry series), and in use on the studio logo for 22 years (Leo has been in use since 1957, a total of 53 years and counting). However, when the MGM animation department, which had closed in 1958, reopened with the Chuck Jones-directed Tom and Jerry shorts in 1963, these shorts used Tanner in the opening sequence rather than Leo, who had already been adapted onto the studio logo and the 1961-63 Gene Deitch cartoon logos.
The lions also have a following on YouTube, with a total of well over 600,000 views.

Slats was the first lion used for the newly-formed studio. He was born at Dublin Zoo, Ireland on March 20, 1919.[3] Slats was used on all black-and-white MGM films between 1924 and 1928. The original logo was designed by Howard Dietz and used by the Goldwyn Pictures Corporation studio from 1916 to 1924 (see left). The first Goldwyn Pictures Corporation film to feature Leo the lion was Polly of the Circus (1917). Goldwyn Pictures Corporation was ultimately absorbed into the partnership that formed MGM, and the first MGM film that used the logo was He Who Gets Slapped (1924). Dietz stated that he decided to use a lion as the studio's mascot as a tribute to his alma mater Columbia University, whose athletic teams' nickname is the Lions; he further added that the inspiration for making the lion roar was Columbia's fight song "Roar, Lion, Roar". Slats was trained by Volney Phifer to growl rather than roar (although in the logo he did nothing but look around), and for the next couple of years, the lion would tour with MGM promoters to signify the studio's launch. Slats died in 1936[4], and his skin is currently on display at the McPherson Museum in McPherson, Kansas.[5] However, the rest of his body is buried on Morristown Road, Gillette, New Jersey, in Long Hill Township by Volney Phifer.

Jackie[2] was the second lion used for the MGM logo. Jackie looked almost identical to Slats, his predecessor. He was also the first MGM lion whose roar was heard by audiences of the silent film era, via a gramophone record. Jackie growled softly; this was followed by a louder growl, a brief pause, and then a final growl, before looking off to one side. In the early years that this logo was used (1928-c. 1932), there was a slightly extended version of the logo wherein, after growling, the lion looked off to one side and returned his gaze to the front seconds later. Jackie appeared on all black-and-white MGM films from 1928–1956, as well as the sepia-tinted opening credits of The Wizard of Oz (1939). He also appeared before MGM's black-and-white cartoons, such as the Flip the Frog and Willie Whopper series produced for MGM by the short-lived Ub Iwerks Studio, as well as the Captain and the Kids cartoons produced by MGM in 1938 and 1939. Despite Jackie's "official" introduction in 1928, the lion had been used on three earlier films: Greed (1924), Ben-Hur (1925), and Flesh and the Devil (1926). A rare color variant can be found on the colorized version of Babes in Toyland (1934), also known as March of the Wooden Soldiers.

MGM began experiments with two-color short subjects in 1927 and animated cartoons in 1930. Two two-strip Technicolor variations of the MGM logo were created for MGM's first color films, with two different lions being used. The first lion, known as "Telly", appeared on all color MGM movies until 1932. The second lion, known as "Coffee", made his debut in 1932, appearing on color films until 1934 (and 1935 for the Happy Harmonies shorts), when production was switched to full three-strip Technicolor filming. The 1934 film The Cat and the Fiddle had brief color sequences, but was otherwise in black-and-white (including its opening credits), so it used Jackie instead of Coffee. (The Cat and the Fiddle however, showed its The End title card against a Technicolor background.)

There has also been an extended version of this logo, seen at the beginning of the MGM short Wild People (1932). It features the lion growling as it did in other films of the time, but lasts a few seconds longer to feature an additional roar by the lion. Then it looked off to one side and returned its gaze to the front a second later.

Logo 3: 1934-1956

MGM began producing full three-strip Technicolor films in 1934. Trained by Mel Koontz (who also trained Jackie), Tanner[2] was used on all Technicolor MGM films (1934—1956) and cartoons (late 1935—1958, 1963—1967), except for The Wizard of Oz, which had the opening and closing credits and the Kansas scenes in sepia-toned black-and-white. The 1941 film Third Dimensional Murder was shot in 3-D and in Technicolor, but it had the opening credits in black-and-white, so it used Jackie instead of Tanner. The 1945 film The Picture of Dorian Gray and the 1949 film The Secret Garden both had brief color sequences, but were otherwise in black-and-white (including their opening credits), so they used Jackie instead of Tanner. (The Secret Garden however, showed its The End title card and the cast list against a Technicolor background.)

Tanner, whose first appearance was before the short subject Star Night at the Coconut Grove (1934) (his first feature film appearance was before Sweethearts four years later, in 1938), was in use as Leo the Lion for 22 years, second only to the current lion (who has been retained for 53 years). It is this version of the logo that was the most frequently used version throughout the Golden Age of Hollywood as color became the norm. The short Star Night at the Coconut Grove, as well as early James A. Fitzpatrick "Traveltalks" color shorts, features an extended version of this logo, with Tanner roaring four times.
Tanner and Jackie were kept in the change from Academy ratio films to widescreen CinemaScope movies in 1953, with Tanner for color movies and Jackie for black-and-white films. The logo was modified; the marquee was removed and the company name was placed in a semi-circle above the ribboning.