I first heard "Helter Skelter" with Mötley Crue as a kid and didn't even know it was a Beatles-song until MUCH later...

Cum on feel the noize I thought was a Quiet Riot original but is of course a Slade-song. I never checked for Slade growing up on the strength of their terrible hits "My oh my" and "Runaway".

Metallica's classic non-album B-side "Am I evil?" is a Diamond Head song but I didn't know until much later for some reason; I guess I was bad at checking credits when I was younger. Also, that song sounded totally metallica unlike many of their other covers...