I have heard from my Rav and during a few derashot of HaRav Eliyahu Mansour that a neshamah previously brought into 'Olam HaZeh in human form, but needing additional tiqqun (i.e. soul correction) and no longer being able to achieve such tiqqun in human form (HaRav Mansour recently quoted an opinion that a neshamah can descend in human form a maximum of three times), can return to This World in other forms (i.e. animal, plant or inanimate form).
I'm no mequbal (i.e. Qabbalist); but, assuming my understanding above is correct, does that mean that every non-human creation is necessarily a gilgul (i.e. reincarnation) of a neshamah that was previously brought into This World in human form?