I recall hearing elsewhere that Discord's current libraries do not support animated WEBP. This is disappointing, as there's a lot of untapped potential in allowing WEBP-format emojis and images.
The GIF format is bulky and inefficient. Orthodox methods of encoding GIFs restrict one to a 256 color palette and 1-bit alpha (basically on/off transparency for every pixel, no variation). To get a reasonable file size from a gif, one must romance with frame dropping and color reduction to achieve a slim yet very ugly end result. However, WEBP images are encoded in a manner similar to YouTube videos (YouTube uses VP9 whereas WEBP is a fork of VP8). With WEBP, users won't need to sacrifice color fidelity or framerate for a tolerable product. Rather, they can simply decrease the bitrate to fill the file size maximum. WEBP also supports full alpha and RGB (even in lossy mode!).
The current upload limit is a bit of a squeeze for GIFs. WEBP compression could prove useful in overcoming that file size limit and creating beautiful high-quality animated images and emojis.