Unexpected Safari Extension -- Download Video
If I had to guess on what this was there would be two things that I would guess that it might be:

Adware


You might have accidentally installed a package somewhere that installed this as an option in the installer, and this would install adware that could change your default search browser. The person who made the extension might have messed up and not made it so that it didn't activate it. 


Real youtube downloader


At one point you might have installed a browser extension if you wanted to download a youtube video. These are everywhere and since youtube does not have a built in free system to download the videos officially, there are a ton of different 3rd party plugins that people have developed and this might be one. If you can remember that you installed it then you shouldn't worry.


Either way, I would uninstall it if you don't use it anymore as it could be anything