I am planning to quickly create a prototype of my game using assets from another game. I am not sure if I am going that way or not. But well I will replace them down the line when I get a 3d artist.
My advice is if you are using copyright assets and releasing the content viewable in public something like a youtube video, never claim its your own work. Always write where you got them from and don’t do anything stupid which will hurt their IP. And never use it commercially. I have seen plenty of other game assets(skyrim, mass effect, minecraft etc) used in ue4 as a testing purpose.