What a hoot! My random googling led me to this:
The War Against the Rull by A. E. Van Vogt
Synopsis: Our hero is Trevor Jamieson, chief scientist of the Interstellar Military Commission, on the front lines of humanity’s war with a shape-shifting race of insectoid aliens known as the Rull. Jamieson may have found the key to victory, but first he must simply survive–marooned on a wild, hostile planet with a 6,000-pound, blue-furred, six-legged, human-hating telepathic bear, Jamieson escapes only to find himself trapped days later in a meteor-carved cave with a woman who wants him dead, armed with only a knife and his wits against a blood-thirsty giant weasel that can claw through solid rock. –Paul Hughes
(Amazing! A 6,000 pound, blue-furred, six-legged, human-hating telepathic bear! A blood-thirsty giant weasel that can claw through solid rock! Talk about B-movie material!)
“IS THAT A RULL SITTING NEXT TO YOU? Now man has conquered all of space and seemingly he is at peace with everone in it. That is, everything except the Rull.
Who and what are the Rulls? They are a race so alien and paranoid that they must have originated in another galaxy. In their true form they resemble some sort of dark, large worm with a variety of appendages. But you almost never see a Rull as he really is. The Rull, you see, can change its outward appearance at will, so that even your closest friend or most trusted colleague may suddenly turn out to be a spy in disguise.”
What to do? What to do? Could my life be in danger? Today is our 3rd year wedding anniversary and he wanted to take me out to dinner tonight but I am petrified so I made some excuse that dinner on a Thursday night won’t be fun because I have work the next day. What if I’m the dinner? I must not let him know that I know that he’s a Rull. Oh, my! I gave birth to a Rull!!! I’m doomed!!!! Humanity is doomed!
But wait… I shouldn’t fear for the Grimm is here! Bwahaha, what the hell am I writing about…
HAPPY ANNIVERSARY, BIG HAIRY INSECTOID CREATURE!