Otter Learning To Swim

Rigby's counterpart Doug Shlibowski from Regular Show episode "Temp Check" is an evil otter that tries to replace him as Mordecai's friend.
In "Alan Dean Foster"'s "Spellsinger" Series of Books, "Mudge the Otter" is the constant companion of the Hero of the Series Jon-Tom. Mudge is foul mouthed, lazy, greed and lecherous, he constantly derides the hero's abilities at Spellsinging, but no matter what he is loyal, courageous and funny.