Originally the French word for horse it is now American slang for a woman whom one dislikes or considers to be malicious or unpleasant because they have come between a once happy and serious relationship.
You and your boyfriend are in a serious relationship, and he suddenly breaks up with you. You find out later that a cheval was the reason for your parting.
When your boyfriend is being an asshole.
You and your boyfriend are currently in different cities and haven't been able to see each other. Following disagreements and being blown off numerous times, you might express your frustration by exclaiming, "He is such a poon!"