As @rahullak states, it's quite subjective how much influence any one person perceives. There ought to be a way to quantify similarity and a threshold for establishing plagiarism. However, the choice of both algorithm and threshold is subjective, so really it's all quite arbitrary. There are Shannon entropy or other probabilistic measures that compare the similarity of two things, but I don't think they can establish similarity by accident rather than by intent. This is one list of algorithms that I found for comparing pictures: https://pypi.org/project/image-similarity-measures/
To be fair, the creature design in Avatar did seem original, although it was obvious biologists had been consulted on the adaptation of the Pandoran fauna to their environment. That the creatures had Earth analogues yet were uniquely different wasn't a surprise.