Saw this in 1979, was supposed to be a double feature...

...For a week in 1979, our local FM rock station KCAL was running ads for a double feature; possibly a midnight showing. The ad promised we could see the first "Marijuana Western." The other flick was to have featured “Beaver the Wonder Dog.” One of the voice-over actors was doing an imitation of Tommy Chong being stoned. So of course my pothead friends and I had to see it. The weekend comes up, and my two buddies and I go to see this in a full theater. I think the three of us found Zacariah kind of boring, it showed some cowboys smoking pot, and that was about it.
The movie ends, the house lights come up and we’re sitting there waiting for the second feature. After awhile, we got the sense that there wasn’t going to be a second feature. Some longhair in the audience loudly, but calm and slowly says “Man, when’s the other movie gonna start? The commercial on KCAL-FM said they were gonna show another movie, the one with Beaver the Wonder Dog, man…” (I recall thinking this was funny, since all of us other longhairs just said ‘KCAL’) and we all agreeing with our share of “Yeah man” and voicing our discontent as well. A few more minutes went by, and then over the PA a voice said “Sorry, guess there was a misunderstanding about tonight’s movie being a double feature. We’re going to go ahead and show The Kids are Alright (with The Who) instead.” So we watched about 45 minutes of that before the three of us got bored again and left. Funny thing is a year later I saw The Who live at the Los Angeles Sports Arena, was blown away and they were my new favorite band for a bit.
I’ve Googled “Beaver the Wonder Dog” since and found nothing movie related and have NO idea what that commercial was talking about. I would have never remembered the title Zacariah, had I not recalled that The James Gang had a starring role.