Cats

Cats are an alien-class specie who live in the Milky Way galaxy who can change their size and appearance at will which they currently use on the humans in order to trick them into thinking that they're harmless pets in order to research the humans as part of the massive project on Earth while sending reports back to Atlantis without detection which they find easy as it's only humans anyway.

Cats assume many forms when on Earth such as lions, tigers, cheetahs, domestic pets and quite a few more forms along with many forms that they haven't used on Earth outside of Atlantis itself and so, the humans never would have seen these forms.

The Cats originate from a distant solar system on the other side of the galaxy where their main colony is and is considered to be an alpha-class specie by some researchers although, this status has not been officially acknowledged.