Black Sabbath may not have been intentionally Progressive but the way in which they constructed several of their songs during the foundational period is heavily influenced by Progressive Rock. The breaking of conventional song structure (verse-chorus, repeat) was already initiated on the first album with the eponymous title track and “Wasp/Behind the Wall of Sleep/Bassically/N.I.B.” albeit the latter was pieced together in a loose, “jam-session” fashion. By Sabotage (if not earlier), interludes became more than merely a well-established part of the repertoire, acting as a counter statement to the precedent (“Don’t Start [Too Late]” vs “Hole in the Sky”). This had of course already been foreshadowed earlier with “War Pigs/Luke’s Wall,” “Wheels of Confusion/The Straightener,” etc. Lengthier epics like “Megalomania” and “The Writ” had progressive tendencies in their development of distinct, separate themes as well though in the case of “The Writ,” how well it’s done is another story. The band I’d say is responsible for the first flowering of Progressive songwriting within Metal.
The Doors were also pivotal in the freeing of Rock structure, “The End” being perhaps the most apparent example there. Tracks like “Crystal Ship” and “End of the Night” shift towards an almost ambient territory while “Light My Fire,” with its elaborate instrumental section in the middle attempted to move away from radio predictability (though to no avail).