In a lie story, you could use the monster with tentacles as a symbol of false fear. The character could lie about the tentacles being able to reach across great distances and invade people's minds. This lie could cause panic among the other characters. As the story unfolds, the truth could be that the monster is actually quite harmless if not provoked, and the tentacles are just for show. This would create a contrast between the initial lie and the actual situation, making the story more interesting.
You can use it as a misdirection. For example, the tentacles could be described as having a certain power, but it's a lie to cover up the real threat. Say the tentacles are said to be able to paralyze with a touch, but actually the monster's eyes are the real danger.
First, introduce the monster with tentacles in a very menacing way in your story. Then, have a character lie about knowing how to deal with it. Maybe they claim that cutting off one tentacle will kill the monster. But as the story progresses, it becomes clear that this is false. You can then build the real solution around something else, like finding a magical amulet that repels the monster. This way, the outline of the tentacle - monster serves as a false lead and adds suspense to the lie story.