The Leaves of Time by Neal Barrett Jr (Lancer, 1971)

 

the gorgon 

came from some time line far across the multiple worlds.  It moved among men, looking always as though it was one of them ... but it was not a man.  For the Gorgon was the destroyer of the universe, and with it came death, and destruction.  The alien could not tolerate other forms of life, and would not permit them to exist.  The first a world knew of the beast in its midst was when its citizens began dying... horribly ... and then it was already too late.  For there was no defense for a beast that could assume the shape and identity of any other creature ... until a man whose world had been destroyed was thrown across the time lines, to an Earth whose only defense was the gentle philosophy of a man long dead ...

a lancer science fiction original never before published

This cover is taking a trip.  An acid trip.  And "The Gorgon" appears to have eaten random capital letters on the back cover.

Cover aside, I  usually enjoy stories about time travel and parallel worlds, and this one is a good example of its kind.   There is a hero, a monster, and an interesting world sideways to this one.  What more could a reader want?