This image below is in the cinemascope 2.39:1 format (compared to the image above in a traditional 1.85:1 "flat" format). There were few animated films made in cinemascope because the image is stretched out to fit on the frame of the film, it passes through a special lens that un-stretches it creating a wide, very cinematic image. Since it was a photographic process making animated films with anamorphic lenses proved to be a very difficult and expensive process. Disney made "Toot, Whistle, Pluck, and Boom," "Sleeping Beauty," and "Lady & the Tramp" in the cinemascope format, there are most likely others but those are the most well known of them.
Cinemascope also presented a couple other challenges compositionally. Having one person on the screen left a lot of space around them, close-ups were awkward, shots had to be longer b/c too many cuts looked too busy and were distracting, and if the lens used was too wide it caused an unwanted vignette. However, those things were easy for directors and DP's to overcome and 'scope (often referred to as anamorphic) is still a popular choice to shoot movies in. Oddly 2D animation isn't, even though now it's mostly a digital process. The only animated film that come to mind made recently in a wide format is "Mind Game" a Japanese film. But Disney's last venture "Princess and the Frog" was made in standard 1.85:1, and Chomet's "The Illusionist" was also in 1.85:1 even though both films were painted, inked, and colored digitally.
( Above and below stills from "Mind Game" 2004)
I have seen a lot of short films and experiments in wider formats but it's weird to me that feature length animated productions (CG is the exception) doesn't adopt it more often. All you really have to do is change the settings and presto! Wider screen, more cinematic venture.