There's good reasons to think that alien species might not look all that different from animals on this planet, simply due to physics. Animals here didn't simply spring to life in their current form; they evolved from single-celled organisms to best suit their environments.
That makes it harder for someone who sees them to convince others that it was aliens and not just a trick of bad lighting or someone with deformities or injuries that give them an unusual appearance.
It also makes it less likely for them to be mistaken for some other species on the planet. Suppose the Federation sent an expedition to ancient Earth that included crew from a species that looked a lot like our cats, and accidentally let Earth people get too good a look at them and their technology.
Those Earth people might think those are Earth cats, and conclude that Earth cats are a lot more powerful and advanced then they thought, and that they had better stop treating cats like animals lest the cats decided to wipe them out and start treating them as superior beings. Next thing you know that entire civilization is treating cats as magical being of great value or even worshiping them as gods.
Or, more likely, the alien "creators" have created them more or less in our image.