[Avodah] Preferred form of maror

[Simon Montagu]

On 2/21/07, Zev Sero <zev at sero.name> wrote:
>  Ivrit makes stuff up all the time; it also borrows
> words from earlier forms of Hebrew, and uses them for completely
> different purposes, especially for concepts that didn't exist in
> earlier times, and so couldn't possibly have had words.  Or did you
> think that Yechezkel knew about electricity?  Or that "chazeret",
> the preferred form of maror, is horseradish?

I was nodding along here until I came to the last sentence. Was
horseradish not known in ancient times? What is chazeret?