Poor monkey! Somebody cut off its paw to make a possibly magical bad luck charm. This critter is forgotten and is rarely if ever mentioned in discussions of the story. But Herbert White does see (or thinks he sees) the monkey's face in the fire the night before he dies in a mysterious factory accident:
[Herbert] sat alone in the darkness, gazing at the dying fire, and seeing faces in it. The last face was so horrible and so simian that he gazed at it in amazement. It got so vivid that, with a little uneasy laugh, he felt on the table for a glass containing a little water to throw over it. (1.66)
Unless you know that the word "simian" means "monkey or ape-like," you might miss the monkey's brief appearance here. Since the face is described as "horrible," we get the idea that our monkey is angry, and either pure evil or out for revenge.
If you were the monkey, how would you feel about people using your paw to make wishes? Would you be mad at that person and be happy to see his wishes turned against him? Do you think the monkey shows itself to Herbert in the fire, or is it just Herbert's imagination?