Equinox [aka The Beast] (US-1970; dir. Jack Woods)
Equinox is a B-monster flick that manages to combine various giant monsters with surreal supernatural adventure, time travel, a Lovecraftian sensibility, holes in the space-time continuum, rampant cavemen, teenagers doing what teenagers do (in these movies anyway) and stop-motion SFX by Dave Allen, Jim Danforth and Dennis Muren. After being unavailable for so long, Criterion recently released it (at a rather high price) in a nice transfer. Here is a trailer:
And here are the stop-motion scenes, conveniently compiled for your enjoyment: