At a physical level, animals have built-in capacities within their DNA as well as those cellular structures and processes that are involved in replication and reproduction, to express diversity in their contribution to the formation of the environment of which they are a part. So, as the environment interacts with them and as they both change within their relationship, we expect there to be changes.
They exist as psychological beings so that the search for the perfect mate, instilled in their “dreams” results in the formation of such creatures such as peacocks. Those aspects of their genetic and epigenetic functioning that are related to a specific image of beauty in the phenotype, are utilized in the formation of that form. Another reason why not to expect uniformity.
Ultimately, we are speaking of particular beings. Organisms that exist as an object in reality as themselves. Each one is a unique expression of its kind, an individual manifestation of a kind of soul which organizes and uses it’s physical components to be what it is, do what it does (instinctively perceive, feel, and behave), to fulfill its particular purpose, ultimately for all to reveal the glory, the power, beauty and creativity that is God. Here too, although there exists an eternal underlying order, one which was brought into existence in a step-wise series of creative events, stopping after the sixth “day”, it would hardly be expected that nothing new would happen after that.