Plantkin come in many different forms. Some identify as a general plantkin while others identify as specific flora. Examples being trees, roses, pumpkins, ect. They may believe that they were supposed to be born as a plant, but their spirits ended up giving them a human body, that their soul is of a plant, or other.
While commonly used by trolls to make fun of otherkin, this is an entirely valid identity as there are people who believe that plants have souls or have consciousness, although not of the same form as fauna. (There is some scientific evidence towards this scattered around. However, there doesn't seem to be much research into this area yet).