Zev Sero asked <<< AIUI, those who've looked into such things (are 
you here R Ari Z?) have concluded that that tradition is wrong, and 
horseradish actually isn't one of the five kinds at all. >>>

RAZ did that one eleven years ago.

Jewish Action, Spring 1996: "What’s the Truth about ... Using 
Horseradish for Maror?" By Ari Z. Zivotofsky

http://www.ou.org/pdf/ja/5766/spring66/LegalEase.pdf

Google is your friend. (This was the very first item when I searched 
for: horseradish Zivotofsky)

Akiva Miller