Manual:$wgMaxAnimatedGifArea

Details
Force thumbnailing of animated GIFs above this size to a single frame instead of an animated thumbnail.

As of MW 1.17 this limit is checked against the total size of all frames in the animation. It is kept equal to $wgMaxImageArea, which is 12.5 million pixels or 3500x3500. In previous versions this was set to 1 megapixel (1000x1000).

Explanation
This means that with default configuration:
 * If an animation is more than 12.5 MP total (width x height x number of frames), you get a single frame when scaled.
 * If an animation is more than 12.5 MP per frame, you get the thumberror when scaling the image.

So if your image pixel size (image pixel size = width x height x number of frames) is higher than 12.5 MP then your image won't work on Wikipedia, when they are scaled. Many of these images who exceed this might appear animated on their image description page because they're displayed in their native size, instead of as a scaled thumbnail.