What is Tamcha?
Probably a type of bitter parsley, specifically gingidion or staphylinus.
Possibly endive.
Not horseradish, or related to it.
A few salient points:
1. Horseradish was not used at all for marror until Middle-age Europe.
2. Horseradish probably did not exist in Israel during Biblical, or Talmudic times
3. Horseradish is not bitter.