enigmatic puzzle - A small mammal?

Small mammal – I'm

A
International animal
With four legs, and,
Whiskers. Sharp eyes.
But, it wouldn't do

To totally give up
A
Answer, too readily,
So those are all the
Clues that I am going
To give you; sorry!

Answer

Impressive construction!

Count the number of characters, including punctuation and spaces, in each line.

18 1 20 20 21 19
18 1 20 20 21 19

Use the simple alphanumeric cipher 1=A, 2=B, 3=C, etc.

RATTUS
RATTUS

You're Rattus rattus or the black rat!