The “green-eyed monster” is jealousy or jealousy personified.
It may have been Shakespeare who coined the phrase and used it in The Merchant of Venice, in 1596.
The phrase could have arisen from the idea that when people are sick, their skin turns a yellow or greenish color. Also, unripe fruit (which will make you sick when you eat it) is green in color. And, of course, jealousy is a sickness.
Perhaps that’s why the emotion is pictured as a monster. It certainly feels like a monster when it has its claws in you. And, because it’s so universal in human nature, it has caused much brutality, including murder. It’s a common theme in fiction.
Jealousy is not the same as envy. If you’re in a relationship and your girlfriend is talking with her ex-boyfriend again, you may become jealous. But if you’ve been working at a company for ten years and someone who just started six months ago gets promoted over you, you would be envious of them, and resent them for getting the promotion.
Envy might inspire you to do better. But jealousy is childish anger because you can’t have exclusive possession of some thing or some person. Both emotions can become more intense and cause much pain and grief.
I’d like to know what the green-eyed monster looks like. Is it a huge dragon with long, long claws and many teeth, belching flame? Or is it my green-eyed cat, pawing at my arm because she doesn’t want me to pet that awful dog?