The jellyfish took no conscious action to attack or interact with Dory and Marlin. They just happened to be in the same part of the ocean, and because they didn't actively do anything, there wasn't much point in having a page on them. It would be the same as if a character stubbed their toe on a rock. The rock just happens to be there and doesn't deliberately leap into a position that allows them to hurt that character. Both are such minor things that they don't need a separate page.