The simple answer to this question would be that magical creatures are capable of using magic as an evolutionary advantage. Magical abilities are often considered to be "wondrous" and "special", but as we know, there are plenty of non-magical animals with unique characteristics like the changing of colours (the chameleon) or biological immortality (Turritopsis dohrnii). In other cases, both magical and non-magical animals share the same ability; for example, the lotus leaf and augurey feathers are able to repel water but only one has a magical origin.
I appreciate that this question can lead to a discussion about how wizards do generally distinguish between "physical" and "magical" phenomena but since wizards don't use scientific methods like non-magical scientists, it strikes me that they would struggle to detect whether a phenomenon is "physical" or "magical" in its nature.
How can they distinguish between creatures that are magical and those that are mundane?