A lot of people don't believe in metal in the 60s, which is fine, but there was undeniably a heavy theme arising in many rock songs towards the end of the decade. I've listed as many as I can think of above but feel free guys to point out stuff I missed, I always like discovering new music. Especially experimental stuff like this.
Btw this is for songs released in the 60s, not written in the 60s, so no Black Sabbath here (unless there's some early EP or demo recoring I don't know about of course).