The Great & Small Quick Start Characters document provides game rule information for rabbits, cats (small), dogs, gopher tortoises, skunks, snakes (constrictor), and squirrels, in large part because such characters begin play at a Size appropriate to the sample adventure, "Secret Of The Spooky Old Warren."
However, the game itself is intended to allow animal characters of any natural size. The Featured Creature series introduces further character species options, and this page consolidates links to them in one convenient place.
Cat, Great (Lions & Tigers)
Fey Pony (MLP-style)
Herd Animal (Bison, Buffalo, Cattle, Etc.)
Horse & Half-Unicorn