This seems like a great idea! Perhaps add a "Night and Day" mechanic, so that maybe only easy/useful monsters spawn during the day, and the big bads spawn at night. That would give the player a break. Some monsters might be:
Some aquatic animals, since you're on an island. Maybe add a a swimming element?
Fish: On death spreads fish oil a few squares away. Maybe fish oil slows down enemies?
Octopus: On death blinds (with ink) nearby enemies for a few turns
Shark: On death, make a spike trap (Take the shark's teeth)
Seal: Amphibious, when killed, gives ability to swim (Use seal skin for flippers)
Blowfish: Poisonous, when killed, gives ability to breathe underwater (allows you to go further from land)(use body as like an air tank)
Frog: Poisonous, on death, poison nearby monsters
Some land animals:
Monkey: Able to jump over traps (Skeleton wall, etc)
Bears: Take 3 hits to kill. Big challenge
Pigs: On death, create a 5 turn wall (with the pig's big body), but doesn't come back to life. Can be set on fire
Pheonix: Flying. On death, sets fire to everything within a few squares.
So far, that's what I can think of.