Name of old horror movie (70s-80s?)

Hi. 

Sorry for the very vague question. 

I am trying to discover the name of a movie that I saw as a kid with my father, on TV. 

I just remember that there was a weird creature loose in a mountain or forest, killing people.

The only scene that I remember is of the creature watching a group of people from behind a tree. 

The creature had a head roughly similar to a human one, but its body was similar to a very thick snake, in a colour like sand. The head was on top of two or three coils of its body. 

Maybe the creature is shape shifter, but I really can't say. 

I watched this movie in the 80s, but the movie could be older than that. In reality, I suspect it was from the late 70s. 

It was a coloured movie, not a black and white one. 

I think the setting was on a mountain where people went in vacation, but not sure.

Sorry for the vagueness, that's all I can remember for sure. 

Thanks a lot for your attention!