Raven, Ice Mephit, Fire Mephit, and Bat familiars all have an inherent damage reduction, beginning at +1 soak 5 damage for all four. For the bat only, this increases to +1 soak 10 damage at master level 5, +2 soak 10 at level 20, +3 soak 10 at level 25, and +4 soak 10 at level 30.

( Source: http://nwn.wikia.com )

"No beast so fierce but knows some touch of pity. But I know none, and therefore am no beast."
William Shakespeare, Richard III