Just in case anybody reads this thread for actual information and gets very confused:
1) There is no Dragon Tank or Dwarf Tank. The boss at water Dragon Isle is the Hi Ho Tank. You don't have to fight it if you don't want the Ice Breath for Mount Pyre. Not getting it isn't really recomended.
2) Dragon Rider is one of Karsh's techs. The enemy I was thinking of is a Dragoon, who is, in fact, mounted on a dragon. He appears in a sideroom in Fort Dragonia. I don't remember if it's an actual starred boss battle--he's only moderately tough--but it is listed as a boss in the encyclopedia. Anyway, he's definately optional, and you don't gain much by fighting him.
Note: Later on in Fort Dragonia, there are weaker versions of this guy walking about as regular enemies.