Who determined the levels:? Rishis, sages, Hindu "theologists."
They're all probably mad, of course. With no more empirical evidence backing them up than their is for the heirarchy of angels proposed by Roman Catholic theologists.
Best pay no attention to either.
A bear can be reborn as an elephant, but not vice versa. They are at different spiritual levels.
An elephant can be reborn as a monkey, tiger, wolf or rat, for example, as these are on the same spiritual evolutionary level. Such lateral moves are common.
Any of these can also progress to a higher level, ie: the one currently represented only by hominids. They will not retrogress to flatworms or bears.
Isn't Hindu mythology/theology colorful? We've got it all over those sober, boring Buddhists.