Sidney the Elephant is an anthropomorphic blue elephant living in the jungle.
Sidney's first cartoon (though he'd appeared earlier as an incidental character in Tom Terrific) was Sick, Sick Sidney, which was also directed by Bartsch and released in 1958, as part of a new wave of Terrytoons. The studio had recently been bought by CBS, and the new artistic director, Gene Deitch (who would later produce a brief but vividly remembered series of Tom & Jerry cartoons for MGM), scrapped the ongoing characters and started from scratch. Gone were Little Roquefort and Dinky Duck; in their place were Clint Clobber and Gaston Le Crayon. But the voices were by studio regulars — Sidney's by Lionel Wilson (Tom Terrific), who also did some of Sidney's pals, and other pals by Dayton Allen (Deputy Dawg) he was back in 1948-1951.
Deitch received most of his training at UPA, the studio that changed the face of animation in the 1950s, where he got his start on Gerald McBoing-Boing's TV show; and so, the Sidney cartoons represented the UPA-influenced '50s style in both appearance and content. The Elephant was both bumbling and, in the spirit of the times, neurotic. The cartoons were mostly about his friends, Stanley the Lion and Cleo the Giraffe, trying to keep him from knocking down the whole jungle.
Deitch's Terrytoons reign was brief, and so were the careers of most of the new characters. In fact, when Mighty Mouse and Heckle & Jeckle returned, in 1959, Sidney was the only one from the Deitch era to remain in production. Over a dozen and a half Sidney cartoons were made, the last few appearing only on TV as part of the 1963-64 Hector Heathcote show. Sidney made a similar impact in comic books, appearing only in the back pages of a few comics where other Terrytoons characters were the stars.
Sidney's Oscar nomination led to a revival, of sorts, decades later. He turned up in a Tiny Toons episode, as a villain. He kidnapped Bugs Bunny and tried to frame Daffy Duck, as revenge for "stealing" his award. (It was Bugs who'd won that year.) But if it hadn't been for that nomination, Sidney would be pretty much forgotten by now.