If Grim Reapers collect souls of humans, than what reaps animals?
Reapers, of course.
Just called Reapers, these animal (like their human counter parts) reap the souls of dying animals.
There are different kinds of reapers depending on their skulls. This one, for example, bears the skull of a Vampire Bat, and reaps souls of all Vampire bats, and any bat related to it. There are reapers of all animals, insects, invertebrate and etc.
They do not carry around Scythes, however as their talons/beaks/teeth are their scythes.

Just a little something I thought of of these guys.
Also, the different coloring is because of the wing membrane.