The biggest vegetable in that movie was the bag boy.
And his 2 genius buddies that egg him on to go outside.
More timid, life forms would probably be frightened away from a dimensional portal. It might be that the more aggressive predators were the first ones to take advantage of the dimensional portal (detecting prey on the other side, maybe provoked by some sort of probe or an explorer that stepped through), or it happened to form in a particularly busy area. A real world example of such an environment would be:
The Cretaceous ocean ranks as the most dangerous sea of all time due to the sheer number and ferocity of its marine predators.
You just have to look at
Hesperornis. This bird spent much of its time on rocky ledges above the water. But it was frequently picked off by small mosasaurs like
Halisaurus, who waited in the shallow caves beneath these ledges for a
Hesperornis to dive in. When it did,
Halisaurus would grab the bird in its short, sharp teeth.
But these mosasaurs were dwarfed by their huge relatives, the Giant Mosasaurs. These gargantuan fiends reached up to 17 metres in length and could take on pretty much everything in the sea.
Also common in the Cretaceous was the largest of the long-necked plesiosaurs:
Elasmosaurus. This 15-metre-long plesiosaur used the length of its neck to sneak up on unsuspecting shoals of fish.
You weren't safe on land either. This period saw the evolution of some of the fiercest land predators of all time, including the notorious
Tyrannosaurus rex.
It's likely that the Arrowhead portal opened onto parallel version of Earth that was in a similar stage of development to the one described above.
I just thought, maybe that massive Hexapod (?) at the end was an herbivore.
I thought the giant at the end was the source of the questing tentacles, like the tentacle that consumed the bag boy....
....it seemed to be covered with tentacles when they glimpsed it.