Animals
Animals have three Attributes: Awareness, Coordination and Strength (as animals are considered to have 0 in the mental Attributes).
Size Categories
Tiny - Max Strength is 1; -2 or more penalty to Marksman skill to hit. Small enough to fit in a boot. Probably this for your Bilge Rat
Small - Max Strength of 4; Children are Small
Average - Max Strength 8; humans, etc.
Big - Max Strength 12; bigger than human, but can still hide
Huge - Max Strength 16; Elephants
Colossal - Max Strength - no limit; +2 Marksman
Skills
Athletics - for running, jumping, flying, brute force, etc
Fighting - obvious
Marksman - used for like spitting acid or a frog's tongue, etc. Not very common for creatures.
Subterfuge - sneaking and hiding, obviously not for picking locks or pockets.
Survival - obvious
Each skill still has Area of Expertise for whatever the animal does best.