Where is the monster on the cover?

Well, the "cover" (presume you mean the video, laserdisc or Region 2 DVD cover) is an adaptation of a promotional daybill poster design which was probably executed before the film itself was actually made. I am pretty sure that the same artist did the original artwork for GALAXY OF TERROR too, which also features monsters not seen in the actual movie.

I will see if I can get Alan to come on here in person and maybe tell us about how the promotional artwork for FORBIDDEN WORLD was made.

I believe you are talking about Mutant(1984) with Bo Hopkins and not Forbidden World, because the creature is defiantly in the later.

The monster on the american poster is not featured in the movie, much like the monsters on the poster from Galaxy of Terror.

The monster on the European poster IS the monster in the movie. It has the head of the original Alien and some spindly spider legs. I'm shocked because the monster in the movie is so crappy and phony looking, I would never have put it on a poster.

The cover was Probably drawn up pre-production.

The movie is also known in Australia, France, etc as "Mutant"

The monster is shown on the cover of "Mutant"

And to be honest if you have ever seen the two covers the original one is better even if its some artists interpretation of the monster instead of the actual one. There is also a "scantily clad woman in peril" on the cover which just adds to its appeal.

You have got to give Roger Corman credit, he has made money making low budget films where many have failed, enticing movie covers like the original are part of that reason...

Kind of typical Rodger Corman marketing. He didn't really worry about accuracy when it came to advertising. Sometimes the posters were done before they had a shooting script, and it was up to the movie makers to do the best they could with a limited budget.