There is a practice in which you fling a rooster around your head three times, utter the phrase "This is my exchange, this is my substitute, this is my atonement. This rooster will go to its death while I will enter and proceed to a good long life and to peace," sacrifice the rooster, and give it to the poor. It sounds like some ancient, pagan practice that is either made up or no longer practiced.
I can tell you that this practice is not made up and is still practiced. It gets even better. It is a practice of Jewish origin called kapparot, which is interestingly neither mentioned in the Bible nor the Talmud. When I first heard of this ritual, I surely thought that someone was pulling my leg. It truly seems like one of those practices that was a part of the past, but much to my amazement, it is still practiced by Orthodox Jews, most of whom are Haredi.
There are multiple reasons to oppose such a practice:
1) Just think about what you are uttering in this practice. "This is my exchange, this is my substitute, this is my atonement." This is a clear-cut example of vicarious atonement. Vicarious atonement is a Christian belief, not a Jewish one. Jesus did not die for our sins and the death of a chicken does not negate one's misdeeds, even if you try to argue that the Hebrew word גבר means both "man" and "rooster." To believe as such would be a violation of some of Judaism's most basic tenets, mainly that prayer and sincere repentance, i.e. teshuva, bring about forgiveness.
2) This practice violates the Jewish value of tza'ar baylei chayim, or the prohibition of being cruel to animals. During Yom Kippur Services, we read G-d's Thirteen Attributes of Mercy. If G-d's mercy is what we emphasize during this time of year, and if being "created in His Image" is done through imitating G-d, doesn't it seem a bit hypocritical to treat chickens in a cruel manner? Normally, I am not one to show PETA propaganda, mainly because I know they have such an axe to grind. However, this footage below best illustrates the practice. Viewer discretion advised.
3) Much like tashlich, the purpose of this practice is to bribe Satan (Machzor Vitri). The Lurian notion that the rooster is Satan himself, and the slaughtering of the roost weakend the demon, does not help. Even if you substitute the chicken with coins wrapped by a handkerchief, which many Orthodox Jews thoughtfully do in order to not violate tza'ar baylei chayim, you still have not removed the superstition from the ritual. Many rabbis, such Rabbi Solomon ben Adret, also known as Rashba, recognized the practice as such, and opposed it for this very reason.
4) Unfortunately, those who hold superstitious beliefs most likely have a low level of education. The reason why I bring up this correlation is because the people who practice this rite will most likely take the practice literally. If one sees this practice as an actual form of vicarious atonement, then one will think that they are cleansed from their sins. This would lead them to believe that they get off scot-free, which means that they think they do not have to do any actual teshuva. This vulgar mentality will detract people from introspection and sincere repentance. That means no personal growth and no progress, which is a true violation of the spirit of Yom Kippur.
Conclusion: Rabbi Yosef Karo, the author of acclaimed halachic text Shulchan Aruch, called this practice a minhag shtut, which is a "foolish or stupid custom that should be thrown out." Maimonides realized that this rite was such a minhag shtut that he never added it in any of his legalistic texts. Customs (minhagim) such as these should be examined with scrutiny. If the practice fits the definition a minhag shtut, such as the practice of kapparot, we not only should take it off the books, but we are halachically able to do so (Tosafot on Talmud Pesachim 51a; Maimonides, Mishneh Torah, Hilchot Issurei Biah; Be'er Heitev, Orach Chaim 182 in Hilchot Birkat Ha'mazon, Orach Chaim 653 in Hilchot Lulav, Orach Chaim 551:4 in Hilchot Tisha B'av).
If I wanted to take an alternative view to make the rite at the point where I would practice it, I would ask for the following modifications: a) make sure that the kapparot is done with money, b) take out any idolatrous elements, and c) make sure the utterance focused on the actual themes of Yom Kippur rather than vicarious atonement or the bribery of Satan.
Since I don't see the "traditional establishment" taking on innovation such as this, I will leave with the following thought. Rather than rely on a chicken to atone for your sins, I ask that all Jews to do the Jewish thing this Yom Kippur by sincerely repenting for what one has done in the past in order to improve on oneself for the upcoming year.
גמר חתימה טובה!