These are often malware or adware.
Xvid is a video encoding format (an implementation of MPEG-4 ASP). Modern Android devices (Android 4.1+, especially 10+) do not need a separate codec download. The system's built-in libstagefright or MediaCodec handles Xvid playback via software decoding. Most "Xvid codec" apps on the Play Store are either fake, outdated, or simply front-ends for a video player like VLC or MX Player. xvid video codec download for android latest version