I wrote a civilopedia entry for the Bilge Rat:
Bilge Rats are huge water rats, grown by Clan Moulder to reach monstrous sizes. In fact, so large are these creatures that their bone structure no longer permits them to walk on land.
Trained since birth to tolerate the troop platform on their back, Bilge Rats are powerful swimmers. Unfortunately, these beasts are unintelligent, and stories of the rats suddenly diving underwater and drowning their passengers are not uncommon.
Bilge Rats are vulnerable to enemy cannon and missile fire, but – if they get close enough – can cause enormous damage by gnawing on the hulls of hostile ships. Attempts to mount offensive weaponry on the troop platform have failed, since the loud noises made by such weapons frighten the Bilge Rat and cause it to become unpredictable and dangerous to the crew.