After a very recent install-from-scratch of Mojave, I was installing a Safari extension and noticed one already there called "Download Video".
It claims to be version 1.0.0 and that it "Downloads video from a webpage". It was not active (i.e. its box in the Extensions manager was unchecked) but I've no idea how it got there. I certainly didn't knowingly do it myself
I did notice one other inactive extension -- "Open in Internet Explorer" -- that I also did not explicitly install myself. However, it says it came as part of Parallels Desktop and I did install that. So that leads me to conclude that the installation of certain apps may also involve installation of Safari extensions. And I certainly have installed some apps since after my clean Mojave install, so maybe the Download Video thing piggy-backed in off one of those? But most of what I've installed is fairly well-known, standard stuff -- MS Office, Emacs, and the like. Nothing I'd expect to have a sneaky video downloader hanging off it.
Any idea where in could have come from?
MacBook Pro (Retina, 15-inch, Mid 2015)