There is an excellent article by Ari Zivitofsky in the Spring 2006 edition of Jewish Action. The article's title is "What’s the Truth about ... Using Horseradish for Maror?"
He talks about the history of maror, when horseradish as maror was first innovated, or else identified as maror:
Horseradish is first mentioned in rabbinic literature by Rabbi Eliezar ben Natan of Mainz (c.1090-c.1170) and the Rokeach, Rabbi Eliezer of Worms (c.1165-c.1230), both of whom refer toand whether it is valid for maror.
it not as maror, but as an ingredient in charoset! Tosafot Yom Tov (Pesachim 2:6) and Hagahot Maimoniot (Chametz Umatzah 7:13) were among the earliest works to identify tamcha as horseradish.
What I consider the most important paragraph in the article:
Heading the list, and presumably the preferred item (according to many authorities, the Mishnah lists these items in order of preference), is chazeret. The Gemara identifies this as chasah, the modern Hebrew word for lettuce, and there is little doubt that the Mishnaic chazeret is lettuce (Lactuca sativa). Lettuce is a winter plant in Israel and thus was, and is, readily available in time for Pesach. Israel’s “wild lettuce” (Lactuca serriola) neither looks nor tastes like the lettuce sold in American supermarkets. It consists of a central stalk with loose, prickly dark green leaves; it continues to grow wild in Israel. The lettuce is bitter, especially as it ages, and when its stalk is cut, it oozes a considerable amount of white, bitter sap. Early cultivated lettuce had this same sap. It might be worthwhile when visiting Israel to seek out some wild lettuce and sample its bitterness.Sounds like something worthwhile to get in time for next Pesach (to eat wrapped up with soft matza ;).