Anishinaabemowin sorts every noun into animate and inanimate, and the sorting is where an outsider should start. Manidoo is animate. So are the sun, thunder, tobacco, and particular stones, kettles and shells: not the class of stones in general, but the ones that have shown themselves capable of acting. The grammar is making a claim, and the claim is that the world contains a great many persons and that only some of them are people.
Manidoo is the word for those persons when they are being considered as powers. It is used of beings of the first rank, of the one that comes to a young person fasting alone and becomes their lifelong helper, of the thunderers who arrive in spring and leave in autumn, and of the beings under the water who are their counterparts and rivals. It is also used of an unlocated quality of power that a thing or a person can have more or less of.
The atlas holds it as a category of its own for that reason. Trying to list the manidoog as a pantheon produces a false object: a table of named gods with departments, which is what a European reference work expects and what the tradition does not supply. What it supplies is a word, a grammar that backs the word up, and a set of practices, the fast, the naming, the offering of tobacco, by which a person enters into dealings with whatever the word covers.
Plural manidoog. The French spelling manitou reached English through seventeenth century missionary and trade accounts and is the form most readers will have met.