Is it possible to pre-download a YouTube video to view it later offline?
Update:

YouTube has removed this feature. Source



YouTube's official app allows you to preload a video. You can either preload all your subscription or the videos you have added to watch later.

When your phone is connected to WiFi and charger, Youtube automatically pre-loads them.

Open Youtube -> Settings -> Preloading

One thing to note is that this still requires data connection to view the pre-loaded video. It won't download the videos but it does something (may be verification) before showing pre-loaded videos.

Read more at support doc