Animated Shields lets you put a moving picture on your shield. Instead of a plain shield, you can show a GIF or image that plays while you hold or block.

Open the mod menu with the default key K, or change that key in Controls under Animated Shields. From there you can browse a gallery of images, clear your current design, change how fast the animation runs, turn the effect on or off, and reload after you add new files.
To add images, place GIF or picture files in the animatedshields folder inside your game folder. The menu tells you where that folder is, and the gallery can open it for you. Choose a file in the gallery and it appears on the red face of the shield.

In multiplayer, your design is shared with other players who also have the mod installed, so everyone sees each shield the way its owner chose it. The mod must be on the server and on each player’s game for that to work. You will also need Architectury API installed.
Simple to set up, fun to customize, and made for showing a bit of personality on the battlefield.