In a review of, let's say, predatory habits of some animals, the subject of the discussion is the predatory habits.
No, the "subject" is "predatory habits of some animals".
subject — topic: something that is being discussed, examined, or otherwise dealt with
There isn't a significant distinction between "subject of discussion" and "objects of discussion". And if you can say a review is of something than it is correct to say that something is the subject of the review (or discussion).
Is it correct to say that the animals, then, are the objects of discussion or objects under review? Or is there a different term that would communicate this?
You can refer to the specific objects you care about: "The animals involved in the studies..."
But any term that could do what you request would have an inherent ambiguity. There is no significant distinction between "predatory habits" and "animals" that would allow a term to single out "animals" as the "objects of review" -- linguistically, they are both objects. The only way to do something like this is to use additional context to inform your reader which of the various objects you are referring to.