There are various apps such as; Install TubeMate, Vidmate or Y2 Mate Downloader These are all popular alternatives for downloading YouTube videos on Android devices. They are all very easy to use and easy to download and install. These apps are not available on the Google Play Store, but can be found online from the various sites. You will only be required to permit your device to install apps from other places, byt this setting might compromise your security. Only download these apps from providers you trust.