Why a GIF from X downloads as MP4
X does not actually store GIFs as GIF files. When someone posts a GIF, X converts it into a short silent MP4 that loops forever. That is why the file you save is an MP4: it is the original file, and it is far smaller and sharper than a real GIF would be.
The MP4 loops in most players and can be sent in WhatsApp, Telegram or iMessage, where it plays like a GIF.
Turning it back into a GIF
If you really need a .gif file, for example for an old forum or an email signature, open the MP4 in any free video-to-GIF converter or in an image editor. Keep it short: GIF files grow very large very quickly.
Saving a X GIF on iPhone or iPad
Open vid-down in Safari, paste the link and tap MP4 HD. Safari asks whether you want to download the file; tap Download. The GIF lands in the Files app under Downloads.
To move it to your Photos library, open the file in Files, tap the Share button and choose Save Video. From then on it behaves like any clip you recorded yourself.
Saving a X GIF on Android
In Chrome or any other browser, paste the link, press Get video and choose a format. The GIF is saved to the Download folder and usually shows up in your gallery a moment later.
If you cannot find it, open the Files app (sometimes called My Files) and look in Downloads, or pull down the notification shade and tap the finished download.
On a Windows PC, Mac or Chromebook
Copy the address of the X post from your browser bar, paste it into vid-down and pick a format. The file goes to your normal Downloads folder and plays in any modern video player.