The quick answer is that you cannot download a standalone Xvid video codec for Android like you can on a Windows PC. Instead, the standard way to play Xvid files on Android is by using a or by converting the video to a more modern format like MP4. Best Apps to Play Xvid Videos on Android
Rather than searching for an external codec pack, install one of these players that include the Xvid codec natively: How to play DivX/XviD videos on your android device how to download xvid video codec for android