I heard that chicken was originally considered pareve in the Bible, because it is a type of fowl (i.e. like fish it is pareve, as it is not 'meat'), and that only later was it changed to be meat because it was too confusing. When did it change to be considered meat? Which rabbi wrote a responsum or takkana making this change, and what was the legal reasoning?
Babylonian Talmud, Hullin, 116a:
So the Talmud is quoting a dispute between Rabbi Akiva and Rabbi Jose of Galilee whether chicken-in-milk is rabbinically prohibited. Both those sages lived in the early 100s. The next story, of Levi and Rabbi Judah the Prince, occurs around the year 200. Clearly by that point the majority practice has become that it's prohibited, but a few towns still follow the minority practice. By the time the Babylonian Talmud is complete in the year 500 or so, the minority practice lives only in the books.
As to the reasoning: some of it may be "we're afraid you'll mistake poultry for beef", but Maimonides actually explains it somewhat differently. "It's easier to tell people no milk and meat together, ever; if you get into the minutiae of what kind of meat, people will start playing games and figuring it only means an animal's meat in its actual mother's milk, which our oral tradition says is false."
As to "which rabbi wrote a responsum", let me backtrack for a moment here to clear this up. Since the Talmud was completed around the year 500, rabbis have not had the power to create new laws out of the blue of the form "don't do X because you may do Y." They may, however, make rules deemed good for the wellbeing of the local Jewish population, e.g. "in our town people are impoverishing themselves with wedding expenses, so we hereby decree that no one may spend more than X on a wedding in this city." (We have lots of these in the history books.) Or a more famous example: "we, the rabbinic leadership of France and Germany, have determined that polygamy is causing familial strife as well as giving the Jewish community a black eye, so we ban it." The role of a responsum is to determine which Talmudic laws, or occasionally later social enactments, apply to the situation at hand.
Note that the rabbis in the 200s still had a formal Sanhedrin that could actually create new laws, we don't anymore. Minority opinions were recorded in the Talmud out of respect, but the ruling followed the majority. So to answer your question -- a majority of rabbis around the year 100 felt it was rabbinically prohibited, and it's likely that prohibition -- like most rabbinic prohibitions -- had arisen over the century or two before that. (There's another opinion in the 100s that it's biblically prohibited, which means clearly some hadn't been doing it at that point!)