Moonrats are strange, large rodents that change under the influence of the moon, becoming stronger and smarter as the moon becomes fuller, until they rival any human in power and intelligence on the night of the full moon.

Aside from being fairly large for a rat, moonrats give no impression that they are different from their more humble kin.

Depending on how strong and smart they are, a moonrat might either try mostly to flee, or to fight rather intelligently. They are equipped with a nasty bite that only grows in power as their strength grows.

3e Monster Manual II (2002)