Symbolism, Imagery, Allegory
Did you notice a trend in Gabriel's descriptions of Finnigan? There's pretty much always an animal in the mix when he does. Here are a couple examples: "Finnigan smiled wolfishly" (7.29), and has "hyena eyes" (7.84). He has "a mind as sleek and slippery as an eel" (9.28), and a "tiger's face in the dark" (3.23). Importantly, none of these animal descriptors call to mind cuddly kittens or playful puppies—there's nothing tame about the animals Gabriel uses to describe Finnigan—and because of this, Finnigan is consistently presented as wild.
This wildness makes sense: Gabriel doesn't recognize Finnigan as a part of himself, instead seeing him as separate, uncontrollable and uncontainable. Finnigan is the name Gabriel uses for the part of himself that cannot be domesticated or expected to play by society's rule. And so when he describes Finnigan, his wildness is ever-present through the references to animals.
It's also worth noting that these animals are all predators. There's nary a vegetarian in sight, which is fitting since Finnigan—though not a cannibal—stalks his prey, he's a hunter just like the animals used to describe him. But he's a hunter gone wrong, one who so consistently fails to follow the rules that—like a rabid animal—the only way to get him under control is to kill him. Which is exactly what Gabriel winds up doing.