I think their eyes are unique.
Macavity
Caracals have unique hunting abilities for cats: you can find more on the link.
Each child in a family is unique by their DNA, physical appearance and personality.
Its lustrous appearance makes it attractive
the saber tooth cats features is that it has big teeth and its claws
dogs cats parrots
silica in the cell wall
The thing that is so unique is that it holds so much tissue and when cats land on their paws they dont feel a thing.
Cats have a very unique way of drinking. When they drink their tongues curl downwards. That is how the cat drinks. It is a very fast process, but that is what happens how cats drink.
because it has a unique appearance like no other bird
She is a Metamorphmagus (is able to change her appearance at will) and is a highly skilled Auror.
The aye-aye has a mix of traits found on other animals, so there is nothing "unique" about it's appearance. However, the combination of traits is unique for primates. They have a fox-like tail, bat-like ears, ever-growing front teeth (like a rodent), and a skeletal middle finger. (The striped possum from Australia also has a skeletal finger, so even this trait is not unique.)