An antagonist is generally a character in a story that opposes the protagonist (usually the main character), although it can also be a force that contradicts the general actions or beliefs of the principal character.
In the Guy de Maupassant short story, "The Necklace," the antagonist would probably have to be Madame Forestier, who loans Mathilde Loisel (the protagonist) the necklace and then rudely comments about her tardiness when she returns it (actually, the genuine replacement necklace). The Loisels then scrimp and save for 10 long years to repay the debts incurred for the replacement.
Homer Barron is the antagonist of Miss Emily (the protagonist) in William Faulkner's short story, "A Rose for Emily." He courts her and then, apparently, refuses to marry her, prompting Emily's drastic method of possessive revenge.
The white hunter is the antagonist in Eudora Welty's short story "A Worn Path." He threatens Phoenix (the protagonist) with violence and speaks to her in a condescending manner.
In all three novels, society in some shape or form is the antagonist.
In "The Necklace", Mathilde's antagonist is her class and social consciousness, stemming from how she viewed herself as a member of society- which is as a very unfortunate person wanting to move forward, but not being able to.
In "A Rose for Emily" social change is Emily's antagonist. The Old South's changes are unbearable to her, and she has determined to not face it and stubbornly keep tight to what she knows which is the past. Change is a huge antagonist, but Social Change is the biggest of them all.
In "A Worth Path" the antagonist is Social Struggle. Phoenix goes through a number of obstacles to get a basic need: medicine. But she is an old, african american woman and her path is obstaculized by a number of elements. All this just to get medicine. Therefore, in this story the struggles of African Americans are represented in Phoenix's own struggle to get to the most simple and rightful place for her to be.
I hope this helps!