The MLP:FiM GIF collection is no longer maintained and updated. You can use this search function or an external search engine to find pony GIFs that were added until season 4. See the animated tag at Derpibooru for a more up to date collection.
Search term examples: Derpy dancing Pinkie eating balloons pinkie pie mane 6 laughing
sonic rainboom equestria girls derpy twilight sparkly library cmc jumping