Poul Anderson 
Trader to the Stars 
Polesotechnic League Book 2

Their space-yacht, pursued by angry Adderkops thirsting for their blood, has run into serious engine trouble. Picking up the trail of another alien spaceship, they decide to board it and force its crew to take them home. But once aboard, its not so easy to find the crew: they’re faced with cages full of bizarre, other-wordly animals: Tiger apes, Elephantoids, Gorilloids, Caterpiggles, Helmet beasts, Tentacle centaurs. One set of these extraordinary creatures must be the crew, in hiding. But which? Survival depends on finding the right answer…